Banks don't care if it's "cheaper than" some other form of power generation, they care that profitability is safe over a very long time span. If power prices crash 5 or 10 years from now because of renewables overcapacity (which is already happening regionally on sunny days with solar), profitability is gone.
If its not a sustainable, profitable business why should a bank invest in it? Banks aren't meant to promote the moral or ethical good. They exist to take money from depositors, replace it with IOUs, and invest the money in ways that are the most likely to be paid back with interest.
That's obviously not to say we can only have industries that work as an open market, but I'd argue that if they don't then the government might as well run the industry rather than fund these half-in half-out companies that appear to be private corporations but only actually exist because the government blessed them.
