The main difference between tablets and phones is the carrier subsidies and nature of the customer. An iPad comes subsidy/contract-free. If you want a tablet, you go out and comparison shop.
When you buy a phone, you're first most likely locked into a carrier (or don't want the hassle of switching). On top of that, the carriers will subsidize your phone to $100, $50, or even free with a 2-year contract. Most non-techie people will say, "Ooh, this has a touchscreen, it plays music, and it's cheap" and go for that one. As many hardcore Android fans there are, there are far many more who don't even know what Android is. They simply buy the right phone for them, and it happens to be a cool-looking Samsung, Motorola, etc.
When you look at all devices shipped (phones, tablets, media players), iOS actually has more marketshare than Android. Only when you compare Android phones to iOS phones does Android have an advantage. Though this seems like comparing weighing all of Apple's lineup against Windows netbooks. Not to mention that there are many other metrics to use (customer satisfaction, revenue, etc.).
