You mean besides putting the extra development effort to make it work on another OS that very few people, in contrast to Windows, use?
Your claim is vacuous until you provide proof that this is actually the reason why they don't offer it on non-Windows systems or allow non-IE browsers to even download it.

Otherwise, judging from their past behavior, this is nothing more than the latest in a long line of anti-competitive and user-unfriendly strategies. Why would they have a sudden change of heart?