Here's why: it's awfully hard to justify asking a commuter in rural Maine to cough up an extra $100 every year to help pay for highway widenings in Portland, when Portlanders themselves don't want them. That's the main reason I would happily join the Tea Partiers to reject any new gas tax proposal that would direct new revenue into MDOT's grubby hands.
The other, larger problem is that gas taxes highlight the urban/rural divide. My wife and I might pay $35-$40 a year in Maine gas taxes. We're middle-class, comfortably employed Portlanders who just don't drive much (and when we do, we frequently save a buck by filling up in New Hampshire). A 10 cent increase in the gas tax would cost us less than $8 a year.
My point is this: if transportation planners want more tax revenue, they need to stop spending millions of dollars on projects that piss people off, and start offering something of actual value.