I do not care too much whether we use MediaWiki or some other tool to track repo requests.
For me, using MediaWiki for them is not extra nice, but it more or less works, and it for sure is good enough™. We do not have 100s of repo requests/day or something. I guess no one likes it, but it's very, very, very seldom I hear complaints about using MediaWiki for it. (And Phabricator would not address these complaints either). And the signal-to-noise ration from the watching of the pages is really good for me :-)
At some point someone took an effort to bring these requests to bugzilla. That never took off.
After some time, it was tried to bring these requests to Phabricator (as MarcoAurelio said already). It never took off either.
I am all for switching to a different tool, if it solves a real-world problem.
Not sure things like "Tables suck in MediaWiki" are real-world problems though, because if $SOMEONE cannot be bothered to copy/paste the previous table row and adjust it for their request, chances are $SOMEONE cannot be bothered to fill out a Phabricator form either.
So the question for me would be: Which real-world problem is solved by switching to Phabricator?