Because too many solo indy wincons are just unfair to read, simply because you can make an indy have literally any wincon.
For example: Wins alone when a day phase begins and there are exactly 10 people alive (13 man game, no nk besides mafia).
It detracts from the game because it means that you can literally understand everything about the person be screwed over by actions that were motivated by a wincon you simply could not be expected to predict.
That's why nobody uses jesters anymore.
So you're suggesting indies should be standardized?
Indies that joint win? Nah, pretty much go crazy unless their expected behavior is openly detrimental to the game.
Indies that win solo? They need indications in the type of wincon they have so town/mafia can actually read for them. SKs and their variants are good in this department. Markers that indicate who they mark are good as well.
Just get creative, you can legitimately figure out their wincon based on the elements of their role/based on how they will have to play in order to accomplish their goal, then this issue will be satisfied.
Nah, **** that, indies are the best. I love nothing more than designing a great indy and seeing great indies designed.
Indies lose their specialness when there's not a single game without an indy these days.