Just to start, I really enjoyed Dawn of War 3 after being sceptical for a long time before release. It was visually impressive, fun and the campaign was actually a great entry into the series.
The thing is, this is the third game in the series and it felt incredibly bare-boned. There are only so many times people will want to wait for their favourite race and pay out for it before they are put off. From an outsider perspective, humans, orc(k)s and elves in space was probably not the most interesting of choices, even though many of us 40k fans know the races are brilliant.
I think there was some inspiration taken from Blizzard for a 3 race base game that would have offered an easier to balance game, but with 40k it truly leaves out what makes the series so brilliant, and that's just how silly it can be. Constantly evolving alien bugs, heretical mutants worshipping powerful gods, japanese inspired space mecha, horny space elves and so on are just far more interesting from an outside perspective. 4 races base would have probably garnered a lot more attention, especially if it was a slightly more out there race. People loved the Tyranids in DoW2 because they had never really been done in an RTS without mods and seeing them in motion and fighting against them was very, very fun.
I hope this isn't the end of the Dawn of War franchise and I wish Relic the best with AoE. 40k interest is still there and there is still an audience willing to be captivated by the brilliant work this studio delivers.