Why the government won't stop subsidizing LPG for cars?
Because that would mean increase in fares for all carros publicos and revolt in the population. The population has had enough of raising costs of all types of fuel and resulting increase in fares for public transportaion (local and long-distance alike). Increasing fares even further would mean a sword over the government's head. So not to increase fares (at least where LPG is concrened) they won't decrease/cease LPG subsidies.
And because it is impossible to say that "carros publicos and private "normal" cars can get subsidiezd LPG", but private "luxyry" cars cannot, (constitution wise, discrimination) they would not be able to institute non-subsidized LPG for luxury cars only.
And besides, the one who can affor Prado and taking it to do shopping around the corner, can affor the prices of gasoline. And the ones thinking, pay couple of thousands more and buy a diesel engine instead.