"What killed VR?"

I think one of the main reasons is because VR is just beginning and so people will be more inclined to wait and see how the tech evolves over the upcoming generations before making a purchase, especially as it is not cheap which is what I think the next biggest reason is.

I think it didn't really died. But maybe VR tech didn't really progressed as fast as people expected it to be, especially in the smartphone world. After the google cardboard was released no real updates followed (well in with respect to smartphones). But I think Daydream can fix that now and besides, other than VR for smartphones, there are VR tech that are still at an early age of progression like HTC Vibe, PSPVR, etc.