Anna is not a party member because she can't fight and she doesn't offer any creative approach as far as gameplay is concerned. Rapunzel isn't a fighter either, but her hair is what makes her a fun and interesting ally.
Kristoff could be a party member but he's too similar to Eugene, so it doesn't surprise me that they decided not to go this route. Having Kristoff but not Anna would also be very awkward.
Elsa could be an obvious and fun party member, but she's probably not available as such because of plot reasons.
Olaf... Why have a tiny, harmelss and obnoxious (I'm biased) snowman if we can have a giant, cool and powerful one instead?
As for BH6, it's even simpler. When the Toy Story trailer came out, there were two common and reasonable concers: "with two more party members, could there be the risk of a too crowded battlefield?" and "with more party members, could fights be easier?".
If we include every member of BH6, that would make 9 people including SDG. The field would be way too crowded, chaotic and messy. As for the difficulty, we'd have two possible scenarios: either it's too easy (because of 6 additional party members) or it stays the same because the party members are so weak they hardly make a difference. But if they're useless, what's the point then?
I don't think that any of this hints at a rushed game. If anything, it leads me to think that it's actually very well-thought.