I'm thinking that "newer" cars either can't be converted to the new fuel source, or newer cars are so shittily made that they're falling apart dead after 30 years, when the cars from 60 years ago are perfectly fine since they were built like tanks to survive forever.
Seriously all this onboard computer shit with "new" cars is not post-apocalypse friendly.
Now lets consider mechanics (greasemonkeys).
Newer cars are harder to fix, that it's probably easier to turn new cars into a houses rather than spending an extra 15 years at a technical college that doesn't exist to fathom all that wiring and fluff.
Also, patents don't mean shit anymore, so when they start building factories to build car parts again, it has to be easier to build a 1950s style factory that builds 1950s car parts... But that solution is probably 20 years away.
Rollers on the other hand are probably built and designed post arcfall and they seem like incredibly basic creatures, that are easy to use and easy to fix.
Are they still using petrol, or are that sticking that glowing blue shit into their gas tanks?