It's because Mando is more more like Boba Fett than Boba Fett.
They couldn't run two shows back to back with literally the same character in a different suit, so they've turned Fett in to something else. It doesn't really work for me, although I did really dig the Sand People episodes.
The last episode kind of stinks of an extra Mando Episode they had no where to put, so they tagged it on Fett.
Likely the Obi-Wan show is going to end up in a similar predicament.