There is no single reason, any in isolation might be OK to regretfully accept but in aggregate: time to quit.
The motto of ATC is Venture, Adventure. A definition of adventure is “engage in daring or risky activity”. I’ve had a lifetime of many and varied adventure activities starting with my time in both Scouts and ATC. I have skills and experience to share. In retirement I felt it was opportune to help do for a new generation what those organisations did for me all those years ago and help deliver a safe introduction to some adventure activities.
It was so difficult to get permissions for outdoor activities that in my roughly 5 years as a CI the only such activities in my squadron were the (supposedly) annual DoE Bronze or Silver expedition.
On one occasion I proposed taking cadets to a local park for a basic navigation exercise. I was told there must be a instructor:cadet ratio of 1:6. A parent was willing to help so we would be able to take 12 cadets - that was not permissible unless they had CRB check and an expedition leader’s certificate, otherwise they could attend but would be treated as one of the cadets, the parent would reduce the number of cadets allowed so I could only take 5 cadets. Furthermore the park featured an ornamental pond where the toddlers float their boats and feed the ducks. This was classified as a hazard and I must ensure that only cadets with an RAF swimming certificate were allowed within 2 metres of the water’s edge. This was probably the most extreme example of objections to proposed activities there were others, but the message I got was the only acceptable level of risk was zero (this at a period over which there were a number of air cadet fatalities in flying accidents).
Contrast that with my teenage years when as a group of 12 Scouts and a teacher we undertook a 3 week backpacking trip, no reams of paperwork just a one page outline plan, a kit list and parental agreement. Maybe that was too relaxed an approach but my recent experience in the ATC was that the aim seemed to be stop anyone doing anything adventurous at all costs.
I was perfectly happy to contribute 6 hours a week to squadron, mostly teaching plus maybe one or two full days a month to take cadets Flying/Gliding/Shooting. As a CI the only reward I sought was the satisfaction of seeing young lives changed so much for the better. That happened but the experiences we were able to offer were so limited that it was rare for a cadet starting at 13 to still be there at 18. Frankly it was boring, most of the time little more than lessons, drill and making (in the cadets words) crappy airplane models. In the end some of the more enterprising cadets organised their own adventure activities without any adult supervision, out of uniform and totally dissociated from the ATC except that the group were all Cadets. I believe my navigation and expedition lessons had motivated them to want to try those skills in the field. I hope the lessons stood them in good stead, however had they been my kids I would have preferred them to have been accompanied by an experienced adult rather than hiking unaccompanied in mountainous territory.
Unlike uniformed staff CIs get no financial support or other form of recognition, I knew that from day one, it was not my motivation and I had no expectations. On the other hand the occasional “thanks” would not have gone amiss. My age meant I was not eligible for a staff role (some might choose to characterise that as age discrimination). I could claim travel expenses, they were not essential to me but I covered significant monthly mileage and the expenses were a help, however administration of them was illogical and unreasonable. I would get home to duty mileage. However if I was collecting a group of cadets from Squadron to go flying I could only claim mileage from Squadron to airfield, not for the mileage from home to the squadron. How does that make sense?
On one occasion I took a car full of cadets to a drill competition at the airfield we used for air experience flights. I was told by the CO that was not eligible for mileage because parents should be doing that - OK but in that case you’d not have had a full drill team there, and how about telling me there’d be no mileage BEFORE asking me to take a car full of cadets?
On one occasion on arrival at the airfield we were told the weather had closed in and there was no chance of flying. So on a subsequent occasion, as the weather was questionable I called the airfield before departing. I was told they were expecting things to improve and there was a good chance of being able to fly. Half way there I got a call from the airfield to say the weather had worsened and there would be no flying so I turned around and brought the cadets back home. On that occasion my mileage claim was rejected by Wing HQ. Should I have continued to the airfield? Or claimed to have done so? By being sensible, saving half the mileage cost and not wasting as much of the cadets and my own time, I had to bear the cost, OK it was less than £30, a relatively affordable sum to me but for others: four times the statutory minimum hourly wage?
I could list more and often do so to the jaw-dropping astonishment of my interlocutors at the crass stupidity of the rules and that there are people who adhere so rigourously to them. Sorry guys but having started as a strong proponent I now find I’m actively trashing the reputation of ATC. The positive outcomes I saw were in spite of the squadron administration, not because of it.
I stuck with it because I did see some very unlikely candidates surpass my and their expectations with great academic results and growing self-worth. I finally left as the result of not just the above but also sundry issues and frustrations like seeing a fellow CI leave because he was being treated with contempt by one the uniformed staff (the others were disinclined to intervene) for no reason I or he could discern (I would characterise it as bullying) and seeing one of the best cadets (a great success at training the drill team, top academic results and very understanding and supportive of the newer cadets) being made to leave for a minor misdemeanour.
It also seemed increasingly the case that the uniformed staff didn’t see it as their role to teach or provide transport to Flying/Gliding, that was CI work. Uniformed staff took part in the opening and closing parade then had an hour or so to do what I’m sure was vital and demanding administrative work 
In the circumstances of an unexpected resignation from a workplace I would expect Personnel department to conduct an exit interview, even more so when dealing with hard to recruit unpaid volunteers. That would serve the dual purpose of helping identify reasons for departure, perhaps an issue that required attention with the opportunity to rectify things or, failing that to at least express gratitude for the volunteer’s past contribution. With ATC: nothing. That’s why I have chosen to write something here.
My conclusion is, regrettably, that this is a failing organisation. To be fair perhaps that squadron or wing is a poor example. They and I will remain anonymous.