it's an OS component, not a normal app
"But if Internet Explorer is any indication, writing a modern application for Windows 8 that still runs on older platforms is a lot harder"
The problem is IE is NOT a userland app, it's source is embedded in the OS and reused all over the place in the GUI. MS have never presented a believable justification for embedding IE this way and pretty obviously only did it for anticompetive reasons - it conveniently became impossible to remove the steaming POS just at the time regulators were starting to ask questions about monopoly abuse.
Once embedded this way, changing the OS layer is guaranteed to bork portability. Perhaps the biggest question is why MS are even trying to backport to Win7, they don't normally miss any chance to drive migration to the new shiny or care if customers get screwed over for not migrating.