I think the volume of the players who would just play Warhammer computer games is a minority compared to the WoW/HS userbase.
Ok, Total Warhammer is good and kind of the exception, while the rest Warhammer games are kind of ripoffs and remakes of the other, dare I say better, games. To go into those games, you have to be at least somehow interested in WH, unlike HS/WOW which are pretty much the leaders (still) in MMO and Card video games. Total Warhammer is kind of the leader in this very specific genre, but it still doesn't get the numbers WoW/HS have.
I can look up Blood Bowl PC game on Steam, and it has 680 in the sequal and 35 in the first game. That is like 700 people...
So I will stand by my point that Warcraft (WOW + HS) has much more users, but Warhammer has more fans who would delve into multiple mediums (toys, table games, computer games, books), the average Warhammer fan must have much higher engagement with the IP than the average WoW/HS player.
But I don't have data, just the intuition that Blizzard managed to become popular with casual users, while Warhammer kind of remains the ecosystem of its own and many people either engage to the high degree, or ignore it.