My guess would be that they weren't included in the original syndication package... and there's no real impetus for anyone to renegotiate to purchase them.
I don't think that's true... for one thing, Pokemon outscores all these shows -- its 380th episode in the US airs on Saturday. There's certainly even good old American shows that utrank Garfield -- the original Teenage Mutant Ninja Turtles ran for ten seasons (and produced 193 episode).
(In terms of how the ordinary Saturday morning cartoon runs, the general idea is that the first season is a small number of episodes because of the expense involved... and then if that's profitable, a second season with a large number of episodes is made with the aim of having enough to be able to rerun the show daily.)