I think Bush is perfectly well aware this is a very good time to top off the petroleum reserve, but I believe the Saudis are pressuring him not to do it. They have been opposed to the Reserve from the beginning, because it gives the US some flexibility and buys us time in the event of a boycott or war. They don't want us to take advantage of low prices to do something that would only make us less dependent on them.

It is also highly likely that these low prices may have been engineered by the Saudis, to both help the American economy and their friend in the White House, but also to help keep demand up. I haven't got a shred of evidence to prove this, but I don't doubt the Saudis told Bush "we'll help you out and lower the prices if and only if you don't take advantage of them to weaken our grip on you."