I wonder if this was to do with insulation? Insulators suitable for 300V will be smaller and lighter than those rated for 600V.
For a tram the rails have to be at earth so the overhead must be at working voltage, just like electrified railways. I did read about problems with stud contact systems where horses could depress and energize the stud with their hooves leading to unfortunate consequences, but perhaps this is an urban myth.
I have really enjoyed following progress of this build. I remember trolley buses in Derby, Reading, Cardiff, Walsall and of course Bradford most;y seen as incidentals to train spotting trips in the 60s. When going to Leeds for my University interview in 1971 I did take a trip to Bradford for a quick ride on a trolleybus. My only experience in the UK. I also remember seeing what I think were ex LT trolleybuses near the railway station at San Sebastian Spain in 1973.