The politicians can't get support for the bill because of the strong lobby of people that get rich over speeding tickets.
Get this story. In DC we have speed cameras now that sense your speed, take a picture, and mail you a $50 ticket. They put them on light poles etc. Well, the company that developed this technology keeps 25% of all the revenue generated by them, which has been in the 9 digit range so far. There's a company that wouldn't be too happy if suddenly all roadways were raised to the 80th percentile speed, it woul cut 75% of their profit.
Small towns and jurisdictions get a MAJOR chunk of their budgets from speed enforcement. The state of NJ for instance would have a huge hole in their budget if suddenly speed limits on the Turnpike were 75-80 vs 65. There are little towns on highways all over america that would have no capital if it wasn't for speed enforcement.
Radar detector companies, talk about a HUGE business that wouldn't be around if suddenly everyone could drive the speeds they drive legally.
So, like anything else the interests with the money have the power. There are several organizations that lobby for higher limits all over the country, and they've been SOMEWHAT successful, but not enough. They did get rid of the national 55 limit.