Well, I don't think that you are going to get an explanation that will satisfy you.
Generally, people either stick with brands that they personally have had success with. They say "I had two WD die in one year, so I only use Seagate." Another way that people can choose is by reputation, which is basically polling other people on the above. The third is by looking at warranty length or MTBF, which are basically meaningless if you are buying one or two units.