Maybe they're delaying his release because one of the following reasons:
1. Baby Mario
It could be that a big surprise could be the addition of Baby Mario. While it surely changes a lot about how Yoshi is played, it could be huge enough to surprise us all. That way, he axed Yoshi, but added Yoshi & Baby Mario.
2. Yoshi's not in it
Sakurai could just tease us, and make us expect Yoshi's in there. Of course, when the game gets released, Yoshi doesn't appear to be in it. What a let-down...
3. Because he's in
Sakurai could just think, they already know about Yoshi, so let's just wait until I run out of filler information. A logical choice. Saving such information could postpone any revealing releases, because you have moveset one, moveset two and Final Smash.
4. He's not done yet
The programmers didn't begin to program Yoshi, or Yoshi wasn't even modeled yet. Maybe they wanted to save programming him, along with other G1 characters, for last, and wanted to start on some new characters.