It's really not an exaggeration to say the Ultramarines movie was just irredeemably awful. It'd be great to get a big-budget, live-action 40k film, but it'd be a huge gamble on behalf of whichever studio takes it on. Because you would need a pretty sizable budget in order to do things justice. Even if you adapted something with less large-scale action, like one of Abnett's Eisenhorn novels, you'd still need to create all the different worlds, costumes etc. which wouldn't be cheap. And then you have to pour this money in and hope it finds an audience. Now, it is hardly impossible, but it is still a risk.
Plus, there is so much lore in 40k, you'd need to find something that can ease the general audience into the universe.