It just... never was something majority actually want so they didn't really get any traction.
Issues wise you also get few nasty cases where you really do not want to keep it with project, like having clients send a bunch of screenshots or even videos of triggering some bugs can grow storage pretty quickly... and while extra few GBs on a file server isn't a big deal, keeping it with code repo just so someone can look at tickets locally is PITA, and you quickly get into "let's not use it, it just makes everything complicated and everyone repo bloated".
Someone could probably implement most of fossil features using git as backing store without all that much problems, the wiki/issues/whatever else features would just be separate, parallel branch hierarchy