It turns out Disney was actually developing a Star Wars Boba Fett standalone film, and the Boba Fett movie got so far as to have a teaser developed.
EW.com reports among the Star Wars spinoff titles considered included a Boba Fett/bounty hunter movie that Josh Trank was supposed to direct. Trank was actually hired for the Boba Fett gig, but is reported to have dropped out following the troubled production on the Fantastic Four reboot.
Originally Josh Trank was to take part at the 2015 Star Wars Celebration and talk about the Anthology films. Disney whipped up a Boba Fett teaser for the event and was going to announce the movie. However, prior to Star Wars Celebration is when things went sour between Trank and Disney, and the Boba Fett teaser was never released and Boba Fett never announced.
Regarding why Josh Trank dropped out of the Star Wars Boba Fett movie, reasons differ. On one hand rumors have pointed to Trank's alleged bad behaviors on Fantastic Four for the reason Disney parted ways, while on the other hand it's reported Disney and Trank butted heads over Boba's Fett creative direction. It's further said that Simon Kinberg wrote the script for Boba Fett, who also wrote Trank's Fantastic Four, and the two didn't get along. Kinberg is also a part of Disney's Star Wars braintrust, which Trank is reported to have disagreed for months with.
What is unknown is why Disney just didn't get a different director for Boba Fett and decided to either pause the project or scrap it entirely. I think it's a fair statement to say that of all the Star Wars characters, a Boba Fett movie is the one most fans want to see.