I think the reason RPM or Proline ''only make parts to vehicles they know they will sell a lot'' is more the fact that a lot of people own Traxxas vehicles and of those people a lot of them want to upgrade. People want to make their truck their own.
Not gonna argue that. Its kind of a 2 fold thing though. To get parts the truck needs to be popular, and the ruck has to have a calling for parts from how it sounded.
like in the MT2s case there was a calling for parts, just it wasn't popular enough for them to bother making the tools to produce the parts.
Now something like the Traxxas MAXX line there is both a need for the parts, and the calling (seriously Traxxas should make the RPM parts stock on that thing according to what I heard from a quite a few owners)
Then again Traxxas is also a popular target seeing if they make one part for one vehicle they can market it to many vehicles. Take the Slash 4X4 majority of the parts are identical on the Stampede 4X4, and now their new rally uses majority of the same parts so that is they now market parts for 3 different vehicles for the cost of designing, and making the mold for one part(actually just looked at RPM's page they have 2 pages of rally parts, all identical to the slash's).
Where most the HPI parts I see they make are the ones that are shared among vehicles(like the savage). Will be honest was kinda shocked to see axial parts, and durango parts being made.