I think Boba Fett as a standalone movie faces 2 problems. The first problem is what I call the 'Spiderman effect.'
If you watched The Amazing Spiderman you've seen it in action. He couldn't keep his damn mask on. This happens for a number of reasons, but two of the main ones are: 1 - Actors don't like to cover their faces; and 2 - It's hard to empathise with a mask.
The Boba Fett helmet is completely non-expressive, so naturally, at every opportunity, his helmet would be removed. This detracts significantly from the mystery of the character.
The second problem is the Boba Fett mythology. It's scope exceeds the value of the character. Some how, (and I have no idea how) a silent background character grew to a status greater than any of the main characters. Now, I've never read any of the EU novels, comics etc, nor have I watched any of the TV shows. So my complete exposure to the character has been the movies, and the massive hyperbole that he's subjected to on the internet.
He's escaped from multiple sarlac pits, he's the baddest dude in the galaxy, he's immune to the force and he flies his own death star.
Even the prequel movies don't seem to have diminished the mythology. Even though now, all I see when I look at him is that ridiculous Kiwi kid.
The character is popular, not because he is an in depth character, but exactly the opposite. He's a complete NON-character. Literally any personality can be given to him, depending on what circumstance dictates. But, the Boba Fett fans, who are the ones this movie would be made for, are fanatical about his EU adventures. And those adventures, are too big for a cohesive movie.
To make this movie would not only cheapen a character that the fans love, it would take a character that non-fans have basically no attachment to and try to build him up to the point of space Hercules. Combine that with the attitude that many people have: The original trilogy are the only REAL star wars movies - and you have something that won't grow the universe, and will in all likelihood, desecrate a beloved non-character.