I think iPhone is at the top of the popularity list purely because of the WOW factor. The apps one can get on it are super cool.
I had the HTC Diamond, and while it was a nice looking phone, it is hugely limited at the moment by its operating system. I tried to find apps and cool things for Windows Mobile but all the sites I went to either looked really outdated or the applications genuinely were outdated. I mean there doesn't seem to be anything NEW and EXCITING for Windows Mobile, whereas the iPhone has a cool BEER application and stylish multi-touch which has been utilised in a very effective way.
I think at the moment, the HTC phones are superb and far nicer than the iPhone in terms of hardware capability, but the Windows and also the Android operating systems still lie behind Apple's.
I would still (and am going to) purchase the iPhone purely because it's great fun to use. It's entirely finger optimised, the operating system is visually stunning and responsive, very responsive, another thing is the on screen keyboard which also is nicer than HTC's offerings.