1. iPhone is not the best phone for most people. A good salesman will try to get you to buy what you will enjoy most and be best for you.
2. Most salespeople work on commission (not BestBuy though) and there is ZERO profit in selling an iPhone. They make profit selling the service plan and accessories that go along with the iPhone, but not the phone itself. Android, blackberry, windows, etc there is the same profit but also profit from the phone. Therefore they make a paycheck.
Reason 2 is probably the biggest reason, since very few people are really familiar with the advantages and disadvantages of more than one OS. When I sold phones for BestBuy it was reason 1 (still would have applied even if I worked on commission)
"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