Partly it's the immediacy. In the long term, China poses a threat to American hegemony, but they're not actively trying to blow things up at this very moment. The other part is the narrative. Environmental disaster should rank higher in peoples' minds, but people are intrinsically drawn to narratives that involve other people. Terrorists are like comic book villains as far as narrative goes. Not only do they behead people, but they brutally oppress women!