I think it also has to do with teh properties of gasoline.

Gasoline requires a much hotter spark to ignite it. And once ignited, is quite hot! Much more so, than glow fuel. And wiht this 'extra punch' you need to start making stronger parts, which might require more material / a beefier part, and, well, more of everthing..

And as stated, you need to have an ignition system, which further adds weight...

I think there is also an issue with machining tolerances, with gas they need to be tighter.