I think at this point Boba Fest has too much baggage, between EU fan expectations and the canon realities of the character. A new Mando allows a completely blank slate (which is what made Boba interesting in the first place).
I think that’s it exactly. As he’d done with so much of the franchise, Lucas milked the character incessantly with backstory of the backstory of the backstory until all mystery and interest had been removed.
The Mandalorian looks like a much needed and wanted reboot of that High Plains Drifter character. I’m signed up and waiting for the first episode.