I expect the fact that traction tyres, along with the minimum possible number of pick ups had a LOT to do with that. How many millions of models of locos must have been made, that only picked up from 4 wheels - two on 1 bogie and two on the other for diesel & electric locos, or for steam 2 on one side plus 2 on the tender. All this regardless of how many wheels were on the loco!
The main makers of British models for a number of years, were Hornby & Lima, who were clearly in the business of making toys, predominately for children. Only later was adding pick ups to more wheels to became a standard practice. Once that was more or less settled, other improvements to the chassis and then the bodies started.