they probably liked alien. keep in mind they also thanked long island iced teas and i don't think that blizzard is going to get themselves into the licqour game any time soon.
Well yeah man, originally Blizzard wanted to make the game based off of the Warhammer 40k universe, but when Blizzard approached the people at Games Workshop, they were being gay and turned down the idea. So in turn Blizzard was like whatever, we'll rip them off then. That ended up turning into one of the best selling strategy games of all time and i'm sure the morons at games workshop regretted their decision back in the day. Yea,they may have released Dawn of War, but like 8 years later and it still is not nearly acclaimed as the 11 year old Starcraft.
Thats why the Terran seem so much like the Space Marines, why the Zerg seem like the Tyranids as you noted, and the Protoss are somewhat (not a lot) like the Eldar.
But in the end, i'm glad things turned out the way they did because now we have two great strategy games to choose from.
The Terran Marines remind me a lot of the Space Marines from 1st Edition 40K - they weren't quite the superhuman warrior monks that they've now become. A lot of the artwork from the original Rogue Trader 40K book showed Marines with slogans spraypainted on their armour, even a few smoking here and there!