In addition to what MKE-MR has said, Hainan is a comparatively less known airline in the US v American or United.
In many ways, the airline industry is a rather price-elastic industry for many customers, where price would usually dictate one's decision on an airline. Being less known, and not part of any (global) alliance, Hainan has less to offer, to put it simply, and hence makes for cheaper prices.
Unless you are part of some sort of a frequent flyer program, I don't think there are many more reasons not to take Hainan. Being so, unless you are part of a Star Alliance frequent flyer program, I wouldn't personally opt to fly United considering their less than desirable business class product on the 777-200.