Robot; Okay. Partly it relates to the history; also in KH1, they actually played a part in the story. (Leon whupping your butt, Cloud in Olympus) Their importance has gone down, but people want to see that the FF part of KH is remembered, mainly b/c there was no real FF in DDD.
As for history; when KH was first came up with, Square and Disney.Japan were in the same office building, hence why the crossover started, so it's always had FF charas as well as Disney. People haven't seen one FF chara yet, so they have been asking about it, since KH has always been Disney + FF from its birth. Also, almost all the FF charas in KH are Nomura designs; Nomura likes to use his own charas. Tho I have been seriously doubting any XV charas are going to show up in KH3; maybe later once Nomura cools off from being ripped off XV.