Money and reputation are the point. Halo is primarily, if not singularly, responsible for the success of XBox Live, a subscription-based system.
Halo + Subscription-only Access + idiots willing to throw down cash so they can hurl base insults at strangers = PHAT CASH.
In an internet-based situation, subscription and therefore access would be much harder to control and maintain.
Halo + Internets + said idiots =/= PHAT CASH.