That's because Samsung throws different models out to see what sticks.
Apple designed iPhone to be useful for most people (as the AT&T 80% iPhone market share shows)
Another way to look at it is; Samsung are catering for a diverse range of customer needs, Apple on the other hand have decided one handset fits all - as long as you're holding it right.
No one would doubt the popularity of the iphone over other products, but not so long ago everyone had a Nokia, and look where they are now...