Hang on, the parent didn't mean a monoculture in the general phone market, they meant one *within* Apple's phones and tablets. The other manufacturers you describe (well not 100% certain about WP7 and WebOS) have, for starters, varying screen sizes. Their phones have different capabilities, in some cases to the extreme. Look at Blackberry - apart from branding, they are more fragmented than Android has ever been, and they are their own singular hardware vendor! For instance, they arbitrarily changed a function name in one version of their SDK which broke even the possibility of forwards / backwards compatibility between those versions. Their phone keypads differ between models, the touchscreen phones don't like non-touch apps and require much faffing about to disable the horrendous "compatibility mode". There is no monoculture within other major vendors.
The fact that Android runs on a huge variety of devices should not be seen as a problem, as properly designed apps will work across all devices, and the Android Market lets you target specific capability requirements anyway! (Disclaimer: If it's not obvious, I am somewhat pro-Android).