The widespread belief is that people who buy electric cars do so in order to protect the planet. That is an important factor for many buyers, but many buyers don't actually care about that, or it's not the driving factor that inspired them to buy an electric car (sorry for the bad pun).
Other factors that pull people in may be a strong desire for energy independence or low-cost fuel. Though, I think these factors again only pull in a small percentage of electric car buyers. There are two other factors that I think have a stronger pull for most buyers.
The first one is something you can't understand until you buy an electric car. It's the instant torque or amazing acceleration electric cars offer. The closest thing I can compare it to if you haven't had a taste yet is a golf cart, except that a golf cart is... just a golf cart.
The acceleration is so quick and fun that many electric car owners get what is called an "EV smile" for years while driving their cars. It's very strong and quick, yet quiet and smooth. The light buzz sound made by many electric cars reminds me of spaceships like those features in a Star Trek. It's a beautiful thing, and it pulls in person after person. There are probably hundreds or thousands of people getting an EV smile right now.
(By the way, this quick acceleration also makes getting onto a highway or into a roundabout easier and less stressful, making driving a safer and more enjoyable experience.)
I think the second most important factor that turns normal people into electric car owners is convenience. With an electric car, you can simply plug in when you get home and then go chill out on the couch or have a meal with your family while your car charges up on its own. In the morning, you unplug and off you go. Forget finding a gas station, getting off the highway, filling up, paying for gas, getting back on the highway, etc. Having an electric car is a wonderful convenience.
This factor may be even more important than the first one. However, I think this is one that doesn't really hit until you actually have an electric car, so it doesn't pull in buyers as much. Nonetheless, it is something electric car owners cherish, and it's not easy to go back to gas once you've gone electric. I've read from numerous electric car owners who occasionally have to drive gas cars how much they hate going back to the gas station, and I've read about big regrets from a few who actually went back and bought a gasmobile. I'm sure their next vehicles will be electric.
I do think the environmental benefits are the most important, but I think it's these two consumer experience benefits that will drive exponential growth of electric vehicle sales. Actually, it seems that it's already doing so.