Many photographers (especially those with full frame sensors or 35mm film cameras) opt for a 50mm prime lens because it is considered 'normal', i.e. not wide-angle or telephoto. Because these lenses are so popular, they are also produced on a relatively large scale, which also makes them cheaper than other lenses of the same speed.
With that said, there is probably a deeper underlying question to be answered:
Why is a 50mm lens considered 'normal'?
There are actually a few factors that contribute to this. If we look at a single human eye from a mathematical perspective (pun not initially intended), the focal length comes out to be around 17.2mm. [as a side note, its aperture is around f/2.1]. Our eye is, indeed, a wide angle lens.
Now, the eye's sensor size (the retina) is smaller than the 35mm film sensor that the '50mm normal' is based on. This will make the equivalent focal length of the eye longer, but not by enough to get it to 50mm, there is another factor at play...
The images that we take with our 50mm 'normal' lens are generally displayed on a screen, or printed (or developed) and displayed on a wall or in an album. Very rarely do we ever get so close to an image that it takes up our full Field Of View (if we did that, we would not consider the image we saw as 'normal' anymore). We generally hold a photo at a distance that makes it look 'normal' (optimally, at a distance equal its diagonal). Because the image only takes up a portion of our FOV when viewed this way, we are adding an additional crop factor, making the equivalent focal length even longer.
Only when we consider all these factors do we come up with a 'normal' focal length value of about 50mm. And remember, that value is only when the image is projected onto a 35mm sensor! for other camera types you need to multiply the magic 50 by your sensor's crop factor to get the 'normal' focal length for your camera.