There's no doubt the population density is greater is sq/m smaller states, particularly to the east coast. Add to the mix that the majority of "broadband" is ADSL/DSL and their inherent "cable lengths" from point of delivery to user and it makes sense that some states with LOW population, large sq/m do poorly.
More than anything is the BUSINESS model in use by ISPs in the USA... bill for everything and the SLOWEST speed we can do so we can charge extra for better access. This multi-tiered pricing is what keeps the USA slow on the web... but is just fast enough that Tom, Dick, or Harry aren't completely and totally pissed at the service.
(After all, these are tele-communication companies which are NOT currently operating under Title II and are fighting it tooth and nail ... ie. Net Neutrality).