What's the improvement of making towniness cases over making scumminess ones on other people, though? Other than being less work in general, I guess.
There are numerous improvements.
1) Making scumminess cases on scum allows scum to respond to those cases and adjust their playstyle and get away with everything.
2) Making scumminess cases on scum is usually not based on confirmed facts, making a towniness case on yourself is. That's why it's much easier to do the latter in a convincing way.
3) For the same reason, it's trivially easy for scum to fabricate a scumminess case on a townie, because it isn't based on confirmed facts anyway so they can come up with whatever arbitrary fake reason and it immediately seems plausible. Fabricating a case that's supposed to be based on true facts but actually isn't is a lot more difficult, and scum probably doesn't want to put the spotlight on their own posts because they know where they have scumslipped and forcing them to make a towniness case on themselves forces them to do that anyway.
4) Towniness cases naturally flow from the rules of the game — I'm allowed to place votes on people for any reason or no reason at all, but I'm not allowed to remove votes that are on me, so I can't demand that people make a case against me before voting for me, I have to make a case for myself being town to convince them to not vote me.