Cos' they didn't decide to just make a typical good game revolving around shooting aliens with a pasted-on plot. They made a whole frickin universe- a story & plot that could've been a best-selling book saga, music that blended Steve Vai with orchestral scores that ended up taking your breath away, and just general attention to detail that puts virtually all other games to shame. Result? A game that has fans coming to a wiki for it to learn about things that have no bearing on the actual gameplay. That's why.
That was an amazing summary ^
Thanks for an answer that states all the reasons everyone should love Halo. ^_^