I think if ED had been churning out planes regularly people wouldn't have cared so much if they did outside stuff, but the long delays on Nevada, EDGE, and any new modern jet since the A-10C I think have people overly anxious. So anything else coming out is pointed to as "time that could've been better spent on X" regardless of whether that's actually true or not.
So the P-51 is a big "why did you spend time on it when you really need to spend even MORE time to get more planes, ground units, and terrain done to make it worthwhile!!!!" when the DCS crowd has really been more about late 20th century aircraft and not WWII.
In other words, your "average" DCS customer is probably more interested in the MiG-21 than a FW190, and more interested in a Hornet than even a 21 (although there are undoubtedly many like me who like it if it flies and shoots stuff. ), so they get really annoyed at the idea that ED is spending time doing WWII birds that delay that Hornet.