The Uber kit includes an iPhone 4S and the UberDriver app is an enterprise "in house" app, not available on the app store. My guess is that the original team used iPhones, preferred that platform and it has stuck. Uber likely buys the phones at a big discount (perhaps refurbs). They want a uniform experience and iPhone is probably the way to go for that.
Why would "conventional thinking" expect them to use Androids?
These are the drivers' personal cars and the drivers' personal phones. That's like saying "conventional thinking would expect Uber would have them all drive a Prius."
I don't know whether or not OP is correct in thinking that Uber drivers' use iPhones only, but I know that for the Uber rides I've been on, the drivers certainly have an extra phone that they use for Uber (I assume company issued) in addition to their personal phone.
