It definitely was considered, and proposed as a follow on stage. No idea why it wasn’t delivered; possibly because the wiring crews moved on to the (much more important) ECML? In snooker terminology it was ‘a bad miss’.
Two things here. Firstly, MetroCentre didn’t exist when the ECML wiring was authorised in 1984.
Secondly, the ECML electrification was proposed by BR originally, to only go as far as Newcastle. That was submitted to DfT for approval in 1983. Whilst it was being considered, the new InterCity sector waded in and proposed extending the wires to Edinburgh, and buying new trains (rather than just locos). Carstairs came later still, and was a fine example of BR accounting engineering. So, arguably, they got extra money for extensions already, and probably didn’t want to push their luck.