As I understand it, the simple technical explanation is that Apple doesn't allow users to install different operating systems on the iPad (well duh, what other reason were you expectingMaybe this is a better place to ask. Why is it not possible to install Android or WebOS on an iPad? What's the technical explanation? I know about the Android port for the iPhone 3G, but that's it?
The flipside is that there hasn't been any tablet OS worth porting over. You asked about Android and WebOS but Honeycomb was only open-sourced last month and webOS is still on its way to becoming an open source project. Windows 8 is still in alpha. Only Ubuntu was available during the iPad launch.
If it were straighforward to install a different OS on the iPad there would be interest in doing so. The device was the one of its kind on the market for over a year. It's less likely now.