The Nexus 4 is a very strong individual model and well known for being a good deal, at least among mobile tech enthusiasts. Samsung, HTC, LG, Sony, et al have a diverse array of phones and only the Galaxy S series has strong brand recognition among the entire Android userbase. But even that has some potential buyers siphoned off by those who prefer the larger size of the Note line.
Also, the Nexus 4 wasn't available for use on either Verizon or Sprint, which is a potential reach of over 150 million customers. Oh, and no LTE on AT&T or official LTE on T-Mobile.