Was it not more that the station building is listed Grade II and alteration on that scale too expensive?
I think it was a case of, lack of space ment that to put it where it ideally needed to be ment the need to alter the building, which as you correctly pointed out is listed. So I'd say it's the former bringing the latter into play, rather than latter being more important than the former.
Or in other words, if there had been enough space then the matter of the building being listed wouldn't of mattered, as the bridge wouldn't of been connected to/gone through/been attached to the listed structure.
Hence why the bridge ended up where the platform 5 buffers used to be ! As it was only the (1 of 2 I can think of) place(s) which has enough room for it to fit without interfering with the listed structure. (The other being at the stops to platform 2, which would of ment shortening platform 2 which was/is more problematic compared to platform 5!)