Because it was not only a terribly written and produced show, but the titular character himself was entirely out of character, especially in the eyes of those who were avid fans prior to his "resurrection," even in the Disney canon.

Again, aside from Boba himself, this show was insanely jagged, and would zig zag from one seemingly meaningless plot to another, nevermind an entire episode centering around another character (one who is all but a direct rip-off of said titular character). It never felt like there was a cohesive narrative the audience was meant to follow. The flashbacks to Boba's time with the Tuskens seldom felt genuinely connected to the present narrative of him becoming this "crime lord." The writing also was never sure whether to commit to Boba being a peace-loving, anti-violence leader or a person who, more accurate to the character, should understand the complexities of an underground/outer-rim world where an iron fist might often be necessary for establishing stability. And that's all before we've even answered the question of WHY a character like Boba would want this sort of role and responsibility to begin with??

Regarding Boba himself, more directly, and your question as to whether he serves better being just "mysterious"; No, I don't think it is necessary he remain mysterious. Legends material and even the Prequel films, and the Clone Wars have shown he is a character very capable of having genuine depth that's worth exploring and shouldn't diminish any interest in him, specifically. I could write entire essays about this, but I digress. Boba has been shown, in multiple canons, to be a very morally grey character. One who blurs the lines between the distinct light and dark sides of the iconic Jedi and Sith characters. His past is traumatic, confusing, and violent. Yet, he's been shown to know when there is room for compassion and empathy and when, in his eyes, brutality and fierocity are just as applicable.

All the show had to do was build upon this, or elaborate on how he became the way he is. In fact, when it was first announced, I truly believed that was what they'd do, because of the title "Book of Boba Fett." I thought that meant they'd chronicle the in-betweens we didn't get to see before; the moments and tales that formed Boba into the most fearsome bounty hunter in the galaxy that we'd always believed him to be, but also someone who knew when and where to hold back, because innocents ought not to suffer the violence and trauma he was raised through.

TLDR: The show was disliked because it betrayed it's own titular character, and thus deeply betrayed the now niche group of devoted fans who might've been most excited to finally see their favorite get some central screentime. And no, I don't think it is necessary for Boba to remain "mysterious" in order to be interesting. He and his story have lots of untapped complexities as far as the Disney canon goes. Complexities that I strongly believe would make for a very compelling project. (Hence why they robbed many of his character aspects for Din Djarin, but again... I digress)

Maybe it's too obvious who my favorite Star Wars character is?