Question

why can concave mirrors not be used rear view mirrors in vehicles?

Answer 1

Concave mirrors form an real and inverted image due to which the objects would appear to be inverted . and also conave mirrors cover a much lesser field of view

Answer 2

Convex mirrors are used in car rear view mirrors as they form a smaller image of the object. Convex mirror can therefore be used to view a much larger area than it would be possible with concave mirror.
Concave mirror on the other hand forms magnified and inverted image of the object. This would form an image of a smaller area and would also inverted which would be hard to understand.
Thus, convex mirrors are used in car rear view mirror.