Why are all hybrids diesel?

I noticed the old buses on No 2 bus route are being replaced by new hybrid buses in my neighbourhood at their turning point on Hayes Place when they head back down to South London. This is of course welcomed in the fight to reduce air pollution from the emissions from the buses as the logo Hybrid CleanAir for London (HV 297 ) indicates but why are all the hybrids diesel? 

So far all the new hydrids on this route l have seen are run on diesel as the cap on the HV 297 illustrates well in the photo below but we do know hydrids can also be run on other sources like CNG ( concentrated national gas ) as in Barcelona, hydrogen in Tokyo or even RNG ( renewable bio-methane ) resourced potential from London’s food waste. Whatever else, maybe even petrol would be better than diesel now that its well established it contributes much more to NOx than other energy sources on the road. 

Some go further and suggest that many in the business know that these diesel hybrids end up running on diesel more often than not. So Transport for London (TfL) need to explain to us why the over reliance on diesel hybrids when other well trodden paths exist with other energy sources in the hybrids, to help us with London’s air pollution crisis.