Now, these are just my personal reasons and may not apply to others, but the two main reasons I don't participate in MAGS as much as I'd want to are:
1. Too often the topics feel a bit too specific, and if I manage to come up with something, it usually so late into the month that there's no time left to make anything resembling a worthwhile game. More broad and general topics that can be interpreted in different ways would make it easier to come up with a fun game concept.
2. The rule about not being allowed to use pre-existing graphical assets. For me, that's probably what takes me the longest to finish when I work on a game, and reusing GUI elements and inventory sprites of generic items would save me a lot of time. It also feels pretty limiting to not be able to re-use character sprites, since I have toyed with the idea of revisiting or expanding upon characters I made for previous MAGS entries, but I just don't have the energy to redo every single sprite for a character I've already worked on, not because I have any good ideas on how to improve the sprites, but just because of a rule in a competition.
I understand the reasoning behind this rule, but I don't think it's necessary. While it could give members who already made games and sprites an advantage over those who hasn't, there's always RON assets or they can ask other members for permission to use their sprites and backgrounds. Now, while there is a possibility that some might try to make entries made entirely out of re-used assets, I still think most will only use it for the things they don't have time to draw and animate before the deadline and still try to make as much original content as possible. The alternative doesn't necessarily mean that all contestants will make good original graphics, I've seen several entries where they ended up using ugly placeholders due to time constraints.