I suspect one reason, besides price, that synthetics are so popular now is because we anglers are like fish: we are attracted by flashy stuff that is new to us. Add in some slick marketing & new materials coming out all the time & we're hooked. Like anything else, fly fishing is susceptible to marketing trends.
I don't mean to say that synthetics are only a marketing scheme. Far from it, in fact. Indeed, many patterns tied with synthetic materials will catch fish often - sometimes even more than the natural materials. But why?
It's because of the colors or the shine or the reflections (read: attractive & attention-getting qualities) of these synthetics. You see, things like hairs & feathers that grow on an animal are almost always round (sometimes they're flat) in shape. That's because they were made for something other than tying flies (at lest from the animal's perspective). This round shape reflects the light in a certain way. How something reflects light determines how the eye see's it.
Many synthetic materials - like those used in carpets or automobile upholstery - look round but in fact are not. They are actually three-sided. They're called "trilobal fibers". Each of the three sides are flat, giving each fiber more surface to reflect light. The color they are dyed determines how much light they will absorb. This is why some are shiny, some are bright, some are very dull, etc. All this extra light bouncing off these fibers (& your fly) is what gives it that flash that catch all the attention under water.
Still, natural hairs & furs & feathers have been catching fish for centuries. Call me a romantic traditionalist, but I prefer the idea of flies made of natural materials. Be that as it may, I like to catch fish. That's why if you look in my fly box or fly wallet you'll see flies with some flash to them, tied with synthetics. I'd like to believe that I'm no sucker for slick marketing, but if it might attract a fish I'm probably going to try it.