I voted 'other' because I think it's a combination of several factors, some of which were listed in the poll.
One: weight. They're lighter and easier to carry, and if people can get the same firepower in a lighter and/or smaller package, they're going to do it.
Two: reliability. Most of the polymer guns I'm familiar with have a good reputation for reliability. I don't think that has so much to do with the polymer, as it does with the fact that they designs are so new that they don't have 50 different companies making clones and mucking with the design.
Three: affordability. Plastic guns are cheaper, and if they do the job reliably, they're going to sell.
I'm not sure where looks factor in. I'm sure some people like the modern looks, but I don't think that very many of the polymer designs are all that good looking. There are quite a few that don't look 'bad,' like the Walther P99 or the FNP, but even they aren't anything special compared to a 1911 or HP. I hate to admit it, though, but the looks of the Glock are growing on me. All these polymer guns are working guns, and not supposed to be fancy, yet the manufacturers try to give them attractive styling, which is hard to do. The Glock is plain and utilitarian looking, because that's what kind of gun it is. It's slick, with no unnecesary adornments (unless you count the finger grooves on the newer ones, but those are functional). I prefer plain jane 1911s, like Colt's 1991 or Springfield's GI, and I think Glock follows in that same vein, and that's starting to appeal to me. The difference, of course, is that 1911s already look good. They don't have to grow on you.