AnthoniusII, I think you have got it wrong. Before the start of the game, there were quite a lot of contestants. After Cassander died, several of his sons became joint king after one of them died of natural causes. However, the were also deposed...
Okay, so wikipedia isn't the best of sources, but I suggest you read a bit of this (
http://en.wikipedia.org/wiki/Cassander), then go to the bottom of the page and hit the next ruler and count the rulers until you get to Antigonos Gonatas. Also count how many were deposed.
Clearly, the Macedonians weren't very powerful during the next thirty or so years, since they were hard-pressed by the Gauls and almost replaced by Epirus. Also, they didn't exactly conquer all of Greece...