I know there is a mountain range and history dating back to WW1; but why does Romania have that distinct difference between the east and south voting left and the northwest voting right?
Is it a case like Poland where the northwest is more developed because it was part of Austria-Hungary or something?
More-or-less yeah.
Transylvania (north-west of the Carpathian mountain range) belonged to Austria-Hungary and had Hungarian and Austro-German settlers and culture (see current President of Romania).
It also benefitted from Austro-Hungarian infrastructure expansion and tourism at the time.
The Wallachia and Greater Moldova at the time were underdeveloped in comparison (and still are).