It's not like the US have been okay with IS's actions up to this point, they've been widely condemning them in fact.
Genocide is a very powerful word, with a very specific meaning. Using it has the potential to completely overblow what IS are doing, making them out to be more of a problem than they actually are. Which is why nobody has been using it up until now.
It's important to keep to the facts, remember Iraq's WMDs? Seems like this time the US is being more cautious, which is definitely not a bad thing.