Reading has not been the county town since Berkshire was split into 6 unitary authorities. Their equivalent locations are now are Slough (lots of passing services), Maidenhead (ditto), Bracknell (I think all trains stop there), Wokingham (as Bracknell), Newbury (skipped by the fast B&H▸ ▸ services) and Reading itself.
I think we are focussing on the wrong issue here. Surely the key factors in deciding how many trains will stop at a station are the size of the settlement, the number of passengers who use the service and its importance as an interchange between different rail services/routes.
Trowbridge, like Northallerton, Dorchester (the Dorset one) or Winchester is not the largest town in its county nor a key interchange on the rail system even though it is the county town. The key interchange points on the
GW▸ system are (I am pretty certain) from east to west - Reading, Oxford, Swindon, Cheltenham, Worcester, Gloucester, the two main Bristol stations, Exeter St Davids and Plymouth. Westbury, Didcot (at least until there's
OHL▸ to Oxford), Bath & Salisbury also probably rank above Trowbridge or any seat of local government in any unitary authority taking all these factors into account.