This can be traced back to stuff Bungie has said in recent years. Along with some stuff you'll find in the community.
1. Halo 1 was a much larger hit than anyone had dreamed. Halo 2 Bungie overshot themselves with ideas and didn't plan properly, they went over budget, over time, and
smacked all the could together into H2 and it showed. Read back through some of the video logs and the staff talks about it in detail. What got cut from H2 they tried to
figure out what was good or bad, and added some of it to H3.
2. Halo 1 is quite moddable, plenty of sites out there for that. Some of the tools were released by one of the companies that helped produce it for PC. Halo 2 has several things
like copy protection and locked out files that have kept it from being modded almost at all. They are almost night and day comparisons. The PC community wishes H2 was open,
but there's no provided tools, and no one succeeded in cracking it open far enough. So it's a dead issue.
3. Microsoft if I remember right, dictated that H2 had to be the poster child for the Games for Windows program, and that screwed a lot up. Far fewer people bought H2 on PC than
on console, plus with no community support due to modding, it fell flat on it's ass sales wise. So MS cut it's losses and told Bungie to focus on the console, as they had done from the
beginning. There was also competition from other much more well known PC FPS titles back then. Essentially the PC version of Halo was Microsoft's spin off... and they killed it themselves.
4. Bungie hasn't been the one to handle any of the H1 or H2 conversions to PC. Even 343 had a limited role in converting CE to CEA, they outsourced most of the work for the update.
5. Halo Wars was Ensemble Studios baby, they just were in constant contact with Bungie for the lore and oversight to make it. It was never designed to be on the PC or in competition to
the PC strategy games. Microsoft wanted a Starcraft for Xbox... and that's what they got. The reason we don't have a Halo Wars 2, is MS closed or bought out... I forget.. Ensemble, and the
team split into 3 different companies.