I couldn't find an answer elsewhere, but having passed through Queensbury yesterday, was wondering why Uno use such a relative backwater as the terminus for the 614/644?
Is it simply that it's the quietest place on the Jubilee Line they could use to serve people living in Brent and Harrow?
It's probably an easier place to terminate with a good connection onto the Jubilee line as well as passing the Northern Line at Edgware with the route it follows.
TFL may also be a factor and it may be that's as far as they are happy for a non TFL route to run.