Hi Rich
Combination of reasons. Lack of compatible RTR rolling stock, the Lima effect, and yes IMHO, shorter wheel base stock will have a broader appeal and wider target market. Another reason may be the 201 prototypes may not have as much nostalgia appeal as the earlier GMs due to the predominant age profile of the hobby, and the very distinctive shape of the older locos with walkways and rails just makes the models more desirable than the plain shape of the modern locos.
That makes a lot of sense re PP where shorter rakes won't look out of place. In IE 121 hauling two or three coaches and a DVT would fit in well on any size layout.
I travelled more miles myself on mk2 and mk3 but bizarrely my 'nostalgia' memory prefers the look, operating fun and great diversity of older stock when playing model trains, but that's just a personal thing. I actually have a 201 but its more for a display case and don't plan to drive it on the layout.