The Arctic route (or, more accurately, 'routes', since there was a northern route in the summer and a southern one in the winter) could avoid pack ice. The Bering Strait route required the use of ice-breakers. There weren't many available and, inevitably, using them considerably slowed down the progress of convoys. However limited use of the route was still made by some convoys formed of Soviet ships. (Perhaps it's because British crews weren't involved that we don't hear much about it?).