I actually think Jon Favreau may have wrote the script originally about Boba Fett.

He mentioned that he started working on the story as soon as he heard about the Disney acquisition (in order to pitch it to them). We also know Josh Trank was set to direct a Boba Fett anthology movie. So since a Boba Fett film was already in development, Favreau went back and decided to make it an original character. But he was still a Mandalorian, since Boba’s connection to Mandalorian culture may have played an important role in the original story and they wanted to still explore that. [Boba Fett’s ambiguous state as a Mandalorian in the new canon may have also influenced this change.]

At some point it is possible that Favreau and Filoni decided to shift the show into a story about new characters rather than old ones. Maybe IG-88 was in the original story, but after they changed Boba to the Mandalorian, they also changed certain characters (like IG-88) into new characters. I’ve also heard that IG-11 gets mistaken for IG-88 within the story itself on a few occasions, which would be a tongue-in-cheek way to address that issue.

So, even after the Boba Fett film was canned, Favreau and Filoni decided to keep the original characters, perhaps because they fell in love with how the story had developed.

But this is all speculation. And maybe if we get a Art of Mandalorian book or something we’ll learn more about it.