I think the Apache's are not able to take being hit nearly as well as A10s, plus have less fire power and take longer to get places.
There's an article here on the problems http://www.slate.com/articles/news_and_politics/war_stories/...
And a story here where Warthogs saved 60 US soldiers in 2013, where "because people were shooting all over the place, the JTAC didn't feel safe bringing in helicopters in to evacuate the wounded personnel."
http://www.af.mil/News/ArticleDisplay/tabid/223/Article/4667...