^^^^
I too reckon that will definitely be for cost reasons. The Forge DV will cost more than the Rev D or G, though I haven't checked what the actual costs currently are. At least, if JKM are using the Rev D in their offered package we can confidently assume that they think it's up to the job. JKM are fiercely independent and like to stand by their products and that's also why they don't recommend the Autotech HPFP internals over the APR pump.
On the D vs G subject, if I understand correctly, R-Tech recommend the G as better than the D but JKM offer the D (unless their web site needs updating).