That was the original reason, but they did say as mentioned above that they are going to bring all the first party Xbox titles to Windows going forward. That means that all future Halo titles (and there will be more) are going to come to PC. I suppose the reason the older ones didn't make it aside from Halo 1 and 2 a while back, is because they have to run on some type of emulation and have never been ported. The Master Chief collection on Xbox One has all kinds of issues that still aren't fixed 3 years later.
So I wouldn't count on any of the older games making the port but I would count on every new title being available day one on Windows. With their Xbox Play Anywhere program, if you buy the game for Windows you get the Xbox One version as well or vice versa if you purchase the game digitally. That's pretty nice and it also offers cross platform play as well. This means if I play on PC and my friend has it on Xbox One I can play with them and with cloud saves I can save my game and go to an xbox and continue the game wherever I left off. Since I have an Xbox One X as well as a PC capable of playing the games, I can choose which version to play. There have been times where the PC version has game breaking bugs so I can simply play it on Xbox until the bugs are fixed and then continue where I left off. For any titles I actually care about from Microsoft this is a very welcome feature for me. The downside is the windows store which has been plagued with issues from download errors that force you to start a download over, to previously locking all your game and app installs to a single folder on the C drive(this has since been addressed). Hopefully there is better support for the PC platform on future releases. There are a couple games that do not support HDR on Windows but do support HDR on the Xbox such as Gears of War 4 and Forza Horizon 3.
Porting games can be an issue.
But not for here.
Halo 1 was also for the original xbox and for windows. I know, i own that game.
Halo 2 for the original xbox.
And the original xbox had a single x86 cpu with a standard nvidia gpu.
Halo 2 was also released for windows once but now is nowhere to be found.
Not even in de windows store. I checked.
It is only for sale for the xbox one in an master chief collection pack.
I think that the original reason still stands for these games.
Halo 3 and Halo 4 are xbox360 games, so there the emulator can be a problem.
Mostly timing issues and not emulating powerpc isa.