Does anyone think this decline has something to do with the nationalization of the oil companies in many countries?
The higher the price goes up, the more likely it is for a government to want to take over the company.
Also the more likely that the oil company that had invested in the venture will have to recover their loses through higher prices.
I think that is certainly a contributing factor. Governments typically underinvest in their infrastructure and do not conduct the long-term R&D needed to extract oil from depleted fields. Governments tend to want to extract as much cash as possible quickly rather than make investments for the long haul. And recovery is much more difficult when a field is depleted.