It takes a certain level of testosterone (or derivatives) to build to a certain level of muscle. It takes less from that to maintain the gains. If you drop below that maintenance level, you are going to lose muscle. Your natural levels arent enough to maintain it after a while.
Its just that simple really, which is why the pros never come off. Other wise you are just cycling on and off, spending your next cycle trying to get up to where you were at the end of your last one, and about the time you get there its time to get off, lose all your gains, and start pct all over again.
Its a frustrating process, and i dont play it any more. I dont go completely off anymore. I just lower the dosage to what it takes to maintain, give my liver a little rest, then jump back in.
Im not saying you should do it; just saying thats what i do now and I like it a hell of a lot better.