That's because there's more addiction potential in cigarettes than e-liquid. Believe it or not, nicotine all by itself isn't actually that hard to wean yourself off of, or at least drastically cut down on. It's one of BT's dirty little secrets: Most cigarettes contain MAOIs -- antidepressants -- that enhance the pleasure derived from cigarettes. That enhanced pleasure makes your brain crave cigarettes even more when coupled with the addictive properties of nicotine itself. e-liquid doesn't have any MAOIs, just the nicotine, so lacking that addiction enhancer you're just left with the straight addictive properties of nicotine, which are much easier to deal with alone.
I was just reading somewhere else on the forum that an ejuice maker is expirimenting with MAOI addition to their juices...I'm thinking that I'll likely skip that. Sorry don't have the link. Came across it in a forum search.