I think the biggest reason behind that is because Traxxas is throwing their weight around big time with money and sponsorships. I've talked to a couple lhs owners, and Traxxas does everything they can to make their product seen on the shelves more than any other brand. At the lhs I race at, the owner says 75% of the rigs he sells are variations of the Slash, and 90% of those he never sees again, other than the owners coming back in to buy parts.
Another point to make, when Traxxas hosts the Torc Series Off-Road races, they'll have RC races also, but ONLY Traxxas products. When we had the Traxxas Shootout last year at the lhs track, the best prizes were for the Slash classes, not the open classes.
If HPI, Losi, or Associated did that, you might see more diversity. Or people would bitch because they were doing it. There's a reason some people despise Traxxas. If they'd back off a bit, it wouldn't be such an issue.