Marginal Revolution suggests greedy government. Mark Thoma suggests that, hey, with ERP roads are a better product so people are prepared to pay more for them. A commenter on the post suggests that it’s actually all to do with our inadequacies as human beings - because with ERP you pay later you tend not to notice so much.
I’m a bit worried about that last one. If true it suggests that prices are not all that good as signals. Which kind of undermines free market theory - or, at least, that bit of the theory that explains why markets work.