The compelling reason is that it's by Microsoft and they are a software company. You can run Windows 8 on hardware that shipped with Windows XP. They are less likely to leave you out in the cold.
Apple stop offering OS updates after a few years, so you're forced to upgrade your phone.
With Android you're lucky if you get one update by your manufacturer and its likely to make your phone unusable. I have an android phone and I'm fed up with substandard software.
I don't have any apple or android devices so I can't comment, but MS has a very long history of making hardware obsolete with their OS upgrades.
In fact it was the Vista disaster that told them people would not play the hardware upgrade game anymore, forcing them to make a faster booting more responsive , less resource hungry Win 7. Some say more so than even XP SP 3 on old machines.
No-one really knows what path they will take as Win 8 matures on phones and tablets- they might just exclude certain CPU's, GPU's, screen resolutions, especially now that they own hardware manufacturers like Nokia and make their own tablets. Its in their interest to force you to update your phone and tablet hardware, just like everyone else does.