Buses also have the doors only on one side usually, if you're just running trams on the surface in traffic you'll probably only need them on the pavement side of the vehicle. It's just got weird in this case the assumption the choice was made on changed after they already had a fleet.

Feels like you're limiting your options quite significantly, though. It's not just underground; elevated trams will often have island stops, and even street-level ones do sometimes.