I just watched
a pretty informative video that dug a bit deeper into the Cedar Fair chain and their investment policies. Basically, money is spent where it can give a return. Cedar Point, Canada's Wonderland, Kings Island. Those are the flagship parks. KBF gets a higher attendance than all three, but it's both completely landlocked and completely crammed, so there's a bit of a limit as to what can be built there.
Michigan's Adventure is one of those parks with an inconvenient location, low visitor numbers and sparse demographics in the surrounding area. A new big coaster there could probably not attract enough new guests to justify its cost, particularly when compared against putting the same coaster in one of the other parks. So it has been consistently prioritized out. Same goes for Valleyfair!, Worlds of Fun and Dorney's. Those four parks make up the bottom tier of Cedar Fair's park portfolio, the "Have Nots" as Airtime Thrills labels them. There's also Gilroy Gardens, but it really seems like Cedar Fair forgot that they purchased that one, so it's not even considered.