Because Bungie was chained down in a first party contract with M$ and Halo 1/2 didn't sell well because they came out during an era when UT, Quake and CS ruled the PC FPS market.
Why would people play Halo when they could play a superior arena shooter with UT and Quake?
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I played the hell out of Halo CE on PC. Never got into CS or UT. UT always was a bit too hectic looking. CS I played a little on a friends PC and felt it was good, but not for me. Never played Halo 2 on PC because it required windows vista at least, and I had XP at the time. That was definitely done to help push people to upgrade.