I've always had the jet bridge with UA in FRA (guess I'm lucky). OTOH I got the remote gate/bus several times when arriving/departing on LH intra-Europe!

When I'm on UA, I'm usually on a flight that arrives very early in the morning in FRA (5-6 AM) and I often depart a few days later on the jet that did the early morning flight, so that may - or may not - be a reason for the jet bridges, as there are seemingly many free gates available before 6 AM (they fill up quickly though).