It all comes down to cost. Aside from the transit buses, most of the other hybrids have been essentially hand built so far, a process that costs a fortune. Even the buses have much lower production volumes than hybrid passenger vehicles. That means little or no economies of scale. Since fleet operators have tight budgets, until the up-front costs come closer to the price of conventional models, the savings in operating costs won't be enough to justify the purchase. Meanwhile manufacturers are reluctant to tool up for mass production unless there is a known market. So we have the chicken and egg conundrum. Without some kind of tax incentives for fleet operators to make the switch, we are unlikely to see a big increase in the numbers of heavy-duty hybrids anytime soon.