The petroleum companies obviously have an alternative source up their sleeves and are waiting until they have bled dry those who will pay for petrol.
I have to take issue with your use of the word "obviously." I don't think there's been enough evidence of a covert alternative energy source researched by oil companies to say it's obvious. And businesses typically don't like to fund R&D projects that will destroy the business's main source of income - even under the guise of keeping it under wraps until their customers are "bled dry."
I tend to shy away from that sort of conspiracy theory thinking, though. And I don't think the oil company execs are competent enough to come up with an alternative energy source, especially while keeping other parts of the business profitable enough for the company to not go bankrupt.
The oil companies want oil to stay popular. The US government has so many ties to oil companies that it also wants oil to stay popular. And besides that, if I were in government, I wouldn't want to risk the sound bites that would surface if American taxpayers were funding spinach solar power. (I'm pretty sure that's the sort of negative spin it would get)