I like Samsung's build quality. I don't need a phone made of metal, and I don't put cases on my phones. If they made phones with Android instead of Touchwiz I would have considered that when I bought my Nexus 4 (which I LOVE the build quality of).
The only reason I won't buy another Samsung phone is because of software not hardware.
The reason they are so popular though is because of marketing. Other than Verizon marketing their Droid line, what other Android phones do you see advertised on TV other than Samsung? There is a reason advertising is a multi-billion dollar industry. People buy what they are told to buy when they see it on TV, internet, magazines, etc.
"Is life so dear, or peace so sweet, as to be purchased at the price of chains and slavery? Forbid it, Almighty God! I know not what course others may take; but as for me, give me liberty or give me death!"
-Patrick Henry, March 23, 1775