I imagine the front of the terminal is a place where groups of people can typically be found - drop offs, pick ups, waiting for baggage, etc.

Also, blowing up the front of the terminal has the potential to shut down airport operations much longer than a remote car park.

It is also to do with glass windows. Terminals tend to have massive amounts of glass which a bomb will blow out massively increasing the casualties. Car parks are open at the sides and sparsely populated.