I imagine it's a problem of internalization of costs and externalization of benefits. The costs are already sunk on fossil fuel energy infrastructure: it's just a clean stream of profits. To undertake switching to green energy, especially as one of the first to do so, a company would have to expend billions/trillions of dollars investing in research and infrastructure that would then likely be used by would-be competitors to enter the market more efficiently and with less overhead. So those first companies, at least absent subsidies or intellectual property ownership, would be externalizing a lot of benefit and internalizing all the cost.