Time for an attack of ACADAMIA!
The reason why Star Wars has such universal appeal can be summed up by the following article.
http://en.wikipedia.org/wiki/Monomyth
For those of you who don't study mythology obsessively (which I'm guessing is most of you)the Monomyth details a kind of story known as the "Hero's Journey" that pretty much appeals to everyone everywhere and always has throughout time. You can make variations on the monomyth but its core assertions are hard to escape.
George Lucas, in creating Star Wars made very deliberate use of Monomythical tropes and devices to give the story such a broad, sweeping appeal.
Here's a quote from ol' George on the subject...
"it came to me that there really was no modern use of mythology...so that's when I started doing more strenuous research on fairy tales, folklore and mythology, and I started reading Joe's books. Before that I hadn't read any of Joe's books.... It was very eerie because in reading The Hero with A Thousand Faces I began to realize that my first draft of Star Wars was following classical motifs"
This focus on Myth and Folklore is REALLY what makes Starwars so endearing, there is a lot of universal appeal packed into it, all dressed up in a fancy new setting that was so different from anything people have seen before.
Halo on the otherhand, while plenty interesting, really lacks this grounding in the Mythic and the folkloric and is, for lack of a better definition, pretty standard Military Science Fiction that uses tropes from Aliens, Starship Troopers and other science fiction works I've never read. As well, the overriding militarism of Halo may make some uncomfortable and the lack of an Everyman Hero may be what prevents it from ever achieving the same status that Star Wars has. Halo also lacks a strong supporting cast, there's no Chewie, no Leia, no Obi-Wan, no Threepio, No Artoo and certainly no Han Solo, it's (pretty much) just the Chief, Cortana and The Arbiter, none of which I find particularly endearing as characters.
Lastly, here's a list of the people whom I am associated with who like Star Wars (Note: Like, as in enjoy watching the movies, not love, as in obsessively fan over)
My Father
My Mother
My Boss
My Brother
My Sister
My Brother-in-Law
My Best Friend
My Girlfriend
Both of my Roommates.
Me.
That is a pretty GODDAMN broad appeal. I simply do not see Halo as appealing to as many people.