KH1 was just about perfect the way he was. Any more characters and worlds - especially ones we now see will be getting attention - would be redundant and overbearing imo :\
There's a reason why we didn't get to see their worlds, at the very least - it would've been pointless.
Now I know KH2 didn't care about that - but this isn't KH2 we're talking about; it's KH1. Every world had its purpose, even if it was to have Alice as the first PoH we meet and have kidnapped. Now that we had Alice though, and Jasmine later on - we hardly needed to go around having PoHs kidnapped in front of our eyes, now did we?
As for their worlds already having fallen to the Darkness - in case you go through Deep Jungle before you get to Wonderland, an alternative scene shows Snow White being brought before Maleficent and not Alice.
So while of debatable canonity (especially in light of a second Riku being in existence should you get through Monstro after you finish Hollow Bastion), it does make one wonder exactly when the PoHs' worlds were attacked.
It also ties in with Beast appearing in Hollow Bastion, following Belle - it doesn't seem like it took that long between his world disappearing and his arrival there - which would indicate it was Riku that was doing the hard, dirty work and thus sped things up for Maleficent.
