Most of the US airlines ordered their 777s in the mid- to late-90s when business is booming. It'd cost them quite a bit to convert to 10-abreast now, and imagine the bad press.

And most the airlines that use 10-abreast use them only on shorthaul (for example, NH's domestic Japanese ones are 10-abreast while longhaul are 9- ) or have generous legroom (like EK's 34"). Probably only airline that has 10-abreast for longhaul without extra legroom is CZ, but they have their own market.

But look at the 787. When Boeing first proposed the plane, everything says it's 8-abreast. But the majority of airline are configuring their with 9-.