Warhammer 40k is awesome because it's the root of everything. Or at least most of the things people care about. The xenomorph comes from the tyranid, starcraft is a blatant 40k ripoff, the idea of elite soldiers in power armour (and even power armour in general) comes from warhammer 40k. Pretty much every sci-fi series or game can be linked to it in some way. Including xcom. Tall psychic blue-skinned aliens who are incredibly powerful yet highly rare? Eldar. Among other things, of course.
Also, it has one of the most expansive and interesting lores in the world. Did you know that, to a non-ork, ork technology doesn't work? The weapons they wield literally don't do anything, the wiring doesn't actually function. They're literally scrap thrown together. The only reason their weapons do anything other than explode is because of their immense psychic powers. They work because the orks believe they work, and their belief literally reshapes reality. Which is an ability they gained because they were originally designed by the Great Old Ones, the most psychically powerful species in the history of 40k.
Also, it has one of the most expansive and interesting lores in the world. Did you know that, to a non-ork, ork technology doesn't work? The weapons they wield literally don't do anything, the wiring doesn't actually function. They're literally scrap thrown together. The only reason their weapons do anything other than explode is because of their immense psychic powers. They work because the orks believe they work, and their belief literally reshapes reality. Which is an ability they gained because they were originally designed by the Great Old Ones, the most psychically powerful species in the history of 40k.