Board Thread:Questions and Answers/@comment-1362555-20140809080356/@comment-167456-20140810144635

Fernando Pandha wrote: Same reason they added Sacks, because they wanted to.

But here's a thought, maybe they will snuggle him in the sequel. Well, they "added" Sacks because of fan backlash at them attempting to whitewash the Shredder. Eric = Oroku, Sacks = Saki. They were originally the same person (and in the 3DS game they still are the same person). A lot of people complained about a traditionally Japanese villain being played by a white actor, so they added Shredder as a separate character and, in the process, turned 'Eric Sacks' into an "original" character.

Since Oroku Saki wasn't planned to appear, Hamato Yoshi might be the same way. But, it might just be because they had so much exposition to get through already that they didn't have space, and they'll save the backstory for later as you've suggested.