MAN and GLA have a base station ,but at one time both did not, so an airport has to start somewhere.
A what now? They're both outstations with no based staff (surely?) and third party handling. A commercial decision has been made to focus on these airports and pull back from others, most recently EDI/NCL/EXT.
Consider TS withdrawing from their monopoly on that famous backwater EDI-YYZ, only for Air Canada to forsake their former base at GLA and re-launch Scotland from EDI with Rouge thrice weekly.
BHX problem was that it didn't have a long enough runway to support Vancouver and Calgary, hence why we could only get Toronto.
Not so much, given the amount of MAN/GLA combos we see, if they'd wanted to serve a YVR-BHX-GLA-YVR it's entirely doable, the runway, as ever, is a red herring. I think these are commercial rather than operational decisions.