Well, if the reason it "worked for them" is because they had promised electoral reform, then they could easily lose their support by breaking that promise.

In this specific situation, the coalition is very fragile without a clear ideological direction. It's very possible that it will break apart, and so it's in their interest to enact reforms that would allow smaller parties to hold influence.

Do you have historical examples to support that claim, btw? You say "a lot of third parties" but which ones?