Since $2017,$ the body of the Ford F$-150$ pickup truck $($the best$-$selling vehicle in North America$)$ has been fabricated entirely from several different Al alloys.

$($a$)$ Why do think Ford made the decision to convert from steel to Al alloys for the F$-150$ body? $($b$)$ In spite of the success of the Al F$-150,$ it is still very rare for car bodies $($traditional sedans hatchbacks, and small SUVs$)$ to be made from Al alloys. Why are the bodies of these vehicles still almost entirely steel?