And not many people want a car that would last for a long time anyway! The main reason is cost. If you want an aluminium car there are a few around, but they do tend to be at the top end of the market. Steel cars don't rust to the extent that they used to, and the main advantage of aluminium is that it is much lighter than steel.
Aluminium is - costly, more difficult to weld/work with and a pretty reactive metal in the right circumstances - intense heat can cause aluminium itself to burn - not good in a crash although the heat of a petrol fire may not be sufficient - not sure. I believe that it is also somewhat more brittle than steel which has design implications re crumple zones etc.