Even though Microsoft says it has a "renewed focus on Windows and PC gaming" internally, we've been hearing that sentiment for a while and little if anything has come to fruition yet. It'd be unexpected for the company to bring more core Halo games to PC -- delightful, even -- so it won't.
Back to reality. Given its focus on Xbox above PC gaming, it makes sense that Microsoft would want to keep its big brand mostly locked to that platform. It's just a shame for us that these and other titles can't also come to PC, even if it means waiting longer than console players.