Why aren't electric bikes mainstream? I've lived in different cities in Europe and Australia, and I noticed more and more the creation of bike lanes, and increases in bike commuters. Still, the number of eletric bikes in the streets is incredibly low. Although it can be said they are expensive (see related question), I cannot believe this is the only reason. 

So I ask the experts: what is the biggest issue to bring them to the mainstream market?
I would add to the already existing answers some of the thoughts that come to me as a bike user.

For me using a bicycle is a mean to prevent getting out of shape. Using an electrical bike would be like going hiking with a car or elevator. This applies for a lot of my friends riding bikes. 

Some of my colleagues complimented me on the fact that I exercise daily by riding to work. In the same register, I expect the opposite reaction for riding an electrical bike, as it would point the user out as a lazy/weak person, unwilling/unable to do any effort. Not saying it's true or right to be judged, but people often live by what others think about them (the coolness factor). you may get coolness points in the gadget section, but it's a different group of people that will compliment you on that. So it depends on where you want to position yourself.

A cheap bicycle is something you can easily toss in some corner when reaching a destination (edit: and of course diligently chain it up), without too much worry regarding theft. Can't do the same (with peace of mind) with an electrical bike.

Depending on where you live, using a cheap (second-hand, maybe) bicycle for two to three months already pays for the bike itself, by saving on transport costs. With an electrical bike, there is a steep initial investment, and the ROI is spread over a much longer term.

Electrical bikes are heavy and not so easy to maneuver in tight spaces where you need to carry them (like stairs). If they are light usually they are very expensive.

Finally there is the same concern as for electrical cars: will the charge last for the whole trip? If not, you'll have to pedal-drive a lot of additional mass accounting for the motor and battery.