Because all your tax money is going to feeding all the benefits for current and former employees, since almost every city loves to make promises about benefits without paying the entire cost out of the existing budget? There's no money for a massive fleet change over, especially since so many cities went and changed over to LNG systems in the recent past.
LNG is good also..I am just tired of hearing all these nature huggers _itching about fossil fuels..wonder what % of our governmental body actually need diesel/gas vehicles for non-emergency use?