Microsoft needs to release up to IE 10 for every currently supported operating system. If Google, Mozilla, and Opera can do it, so can Microsoft.
They perhaps "could", but they probably won't. Their current policy is only to support new browser versions for the latest two MS OS's... which means Vista and XP will be out of luck for IE10 and above. Every company has to draw their own line about how old an OS they will support when releasing new products, and that line for free software like IE will understandably be drawn closer to the latest OS version(s) when the company also is in the business of encouraging migration to the newest OS version that they're selling.