A) they are unproven. Many people believe that something that isn't proven safe or unsafe is not better than something unsafe that has been heavily scrutinized and studied. What if there's a much worse effect that we dont' know about yet?
B) They are unstandardized. Each producer can be putting who-knows-what in the oils.
C) It's still smoke. When they first came out, a lot of people got e-cigs with the idea that they could get around smoking bans, such as on planes or in bars (in places that banned that). This has resulted in a massive backlash, because it really wasn't health problems that caused those bans, it was the fact that nobody wants to sit in someone elses smoke cloud. This is causing a big social stigma against e-cig users.