It's all about cost, look at CNG.
I believe the cost to convert is cheaper since the gov don't need
to built CNG filling station like for CNG car.
Maybe can just leave the solar panel at your parking lot to
absorb the solar energy. And should be cheaper in the long run, if
the initial cost is not extremely expensive...
And the gov can help to reduce cost by maybe lowering the tax
for solar powered car (which unfortunately is not the way our gov
is run).