But as far as why people keep buying the products: sadly it's because they're better. I'm looking at the GS4 (i9505, not the Exynos one) right now vs. the HTC One and Xperia Z. On paper, they're all basically the same phone (2G quad core snapdragons with 1920x1080 screens). But all the tiebreakers (really, all of them) go to the Samsung phone. The GS4 is slightly smaller, thinner, and lighter yet its screen is slightly larger. It's CPU is clocked higher and its battery is larger. It's cameras are better (much more so than the HTC, slightly more so than the Sony). It has a removable battery where the others don't. It has a sdcard slot where the HTC doesn't. It supports the "T-Mobile" HSPA+-on-AWS-band frequencies. It has a thermometer, hygrometer and barometer built-in.
It's just a better phone on everything quantifiable. Not so much so as to kill the competition, but to anyone paying attention and recommending hardware to their friends, they're doing an awful lot right.