Some e-cigs are poorly made and the rechargeable batteries can and do explode.
For this reason airlines / regulators do want passengers to carry them on. Better in the cabin where the explosion / fire can be detected and dealt with, than in the hold where it can not.
Doesn't this make you feel safe and comfortable.
The nicotine liquid inside is toxic. A broken e-cig will leak. Anyone exposed to the liquid can become very ill.
Who says the aircraft detectors can't detect e-cig vapour? Older ones cannot but technology does move with the times. There are components of the vapour that can be easily detected in a closed environment like a lavatory.
Yet another example where addicts' habits are more important (to them) than aircraft and passenger safety.