Because cigarettes deliver nicotine very, very fast, giving an instant and pleasant rush. After an hour or so the nicotine level in your blood has fallen quite a lot and you'll be craving another one.

E-cig nicotine delivery happens slower and you avoid both the highs and the crashing lows. That makes it a lot easier to cut down to insignificant amounts of nicotine over a few months.

Tobacco smoke also contains monoamine oxidase inhibiting compounds, which are believed to drastically increase the addictive potential of nicotine. Rats are largely indifferent to nicotine alone, but will compulsively self-administer nicotine in combination with an MAOI.

http://www.sciencedirect.com/science/article/pii/S0024320506... http://jneurosci.org/content/25/38/8593.long http://www.sciencedirect.com/science/article/pii/S0091305703...

Cigarettes do give a more immediate and intense nicotine 'hit'. Ammonia compounds are added to cigarette tobacco, which increase the proportion of freebase nicotine to nicotine salts. Cigarettes are, in a very literal sense, the crack cocaine of nicotine delivery.

http://www.ash.org.uk/files/documents/ASH_623.pdf

In a lot of drugs, euphoria is at least partially proportional to the rate of blood level increase, e.g. [1].

[1] http://www.dddmag.com/article/2009/07/question-opioid-euphor...

As other have said better than I could have - analogs are more of a one size one dose kind of thing.

PLUS To change your dose you have to change your habit. With Ecig, to change the dose you just buy a different product, but your day to day (hour to hour) habits are the same as always.