I'm not sure why, patents and strong-arming royalties out of car companies and end buyers everywhere just because you can aside, 1234yf is used instead of plain, old propane. R290 gets the same job done and doesn't cost an arm and a leg to use...
That said, R290 has been around forever and there are huge debates as to whether or not it actually increases the risk of fire in case of an accident. There are many who will retrofit their AC compressors to use R290 instead of the now significantly more-expensive R12 or its incompatible and inferior-cooling R134a alternative.
The argument goes that R134a is warmer enough that it takes less R290 to do the trick. It's a closed, separate system, and the gas itself is largely distributed and not pooled (like a gas tank would be) and there isn't enough of it to actually sustain a fire. It's also claimed that it's simply not as easy to combust as it's made out to be, but I'm not a chemist and I don't know.
That said, R290 has been around forever and there are huge debates as to whether or not it actually increases the risk of fire in case of an accident. There are many who will retrofit their AC compressors to use R290 instead of the now significantly more-expensive R12 or its incompatible and inferior-cooling R134a alternative.
The argument goes that R134a is warmer enough that it takes less R290 to do the trick. It's a closed, separate system, and the gas itself is largely distributed and not pooled (like a gas tank would be) and there isn't enough of it to actually sustain a fire. It's also claimed that it's simply not as easy to combust as it's made out to be, but I'm not a chemist and I don't know.