My theory is it is a combination of dehydration (new vapers often overlook the importance of hydration) and sensory overload. It is a known fact that your brain will simply filter out input from certian senses (taste, smell, hearing) when the input is constant, repetitive, and strong.
Because cigarettes are inconsistant in their manufacture and burning is a natural, unpredictable process, every puff from every cigarette is different. Smoking a cigarette is like listening to a "distracting" converstion while you ride the train--you might want to tune it out, bu tyou cn't help yourself but to listen and pay attention.
Vaping is much more consistant, flavor-wise. Plus it has lots of NEW strong flavors, that just keep coming at you--the same every time. So Vaping is more like the annoying buzz of an air conditioner. You notice it when you first walk into a room, but after a few hours, you don't even notice it conciously. Your brain has filtered out the noise.
And because that consistent delivery of strong flavor is something your brain doesn't really know how to process, when you are a new vaper, once the novelty wears off (usually round week 2 or 3), your brain just starts to filter out those flavors--ESPECIALLY if you vape the same thing all the time.
When I started vaping I was all over the place, flavor-wise. I tried tobaccos, fruits, bakery flavors--and I never got "vapers tongue". It wasn't until I found a flavor I REALLY likes--about 3 MONTHS in--and started vaping it 24/7, did I wake up one morning and the flavor seemed to have all just disppered--and it was like that for about a week, while I frantically went through my entire juice collection, looking for a flavor that would stimulate my tastebuds back to life. Ultimately, I think it's just a matter of training your brain to NOT filter out the "flavor overload"...