They have been nerfed (compared to 09), but I average about a unique per full 8-player game. You may be getting some failed uniques, which happens when unique rolls and there is no unique for that item.
Either way, that is about average. If you kill all the unique cows in the level you have a lot better chance for unique items, just as it is with any area of the game.
It's just bad luck. I have a little more mf and some runs I find 5 uniques sometimes none. Though if you kill the king he seems to drop pretty darn well for me. He seems to drop like bugged andariel. Though I still haven't gotten any useful uniques off of him.
8 runs doesn't seem much to me.
Most unique items are dropped from bosspacks and according to Levels.txt, there is 6-8 bosspacks in Cow level, which is quite low number in comparison with number of regular cows. For example 6-8 bosspacks spawn also in Pit lvl1 or Ancient Tunnels which are much smaller areas and can be run faster. Also with my javazon (who has similar MF), I have to skip around 1/3 to 1/2 cow bosses, because they are LI.
Since there are a lot of regular cows, the player number is more important there.
Cows are not famous for dropping uniques, they are famous for dropping "goods" - gems, charms, jewels, runes... and socketed items.