Why M939s instead of the FMTV?

I was wondering why the devs chose to use the M939 over the newer FMTVs for the US transport and logistics trucks?

They seem antiquated next to the nice new up-armored M1151. When I think M939 I think the old cloth top HMMWVs Like the M1038s.

If I had to guess, maybe there just wasn't enough model data on the FMTV whereas the M939 has probably been done to death.