Mobiles phones is recent days have become an extension of your own persona for a lot of people, hence the zealot-like behavior of many.
Especially in the US, were mobile phone contracts are insanely expensive, and in a lot of situations your aren't even free to use a separately bought sim-lock-free mobile, without paying extra.
A lot of customers view mobile phones and consequently mobile OS-es as a status symbol and/or fashion statement.
Most people haven't really used alternative mobile phones because of the costs. At least the fact that Android users are visiting this site means they are interested in an alternative. So maybe they will collect enough courage and actually try something new.

There are a huge variety of mobile phones, mainly because of the different operating systems. One phone may be the best fit for your specific daily usage, or offer the best costs to features ratio, but they all have their flaws. If you can't see anything where other phones are better at, your judgment is clouded, or you just don't know enough about the competitors.