You'd like to think that a company like JD could have persuaded DD or Cummins to enter the market with their projected sales of GTs and SCUTs.
Some #'s from last Fall:
Compact Tractor Sales Strong in October Across North America | 2 17-11-14 | Rural Lifestyle Dealer
For a given hp engine range, I don't think any of those USA #'s are high enough (all manufacturers combined) to distract DD or Cummins from their primary large products. (What I do find weird about those tables is how low 4wd sales are vs. 2wd).
Given the hoops off-road manufacturers now have to jump through emission wise, the numbers have to be pretty large to justify a new engine range.
Rgds, D.