I think you'll find that its not very popular due to its strongish taste. The same can be said for ox-liver too. Generally the stuff sold in butchers and the like is either chicken or lamb's liver. Everything else gets used in pate dishes, goes for mixtures in pies and stuff or gets turned into canned dog-food. The nutritional aspects of liver are very good. Its probably one of the best sources of bio-available iron going with the exception of black pudding possibly (which uses pig's blood).
Don't eat polar bear liver however, that one is pretty toxic due to the high level of vitamin A contained in it. Anywhere between two to four ounces can easily kill.