Nobody has yet mentioned that jet intakes run into problems feeding the
engine properly at negative angles of attack.

So the reasons are...

1) aerodynamic (inlet at negative aoa)
2) flight dynamic (can generate positive pitch rates greater than negative)
3) structural (can pull more positive g's than negative)
4) aeromedical (pilots can handle positive g's better than negative g's
5) situational awareness (during positive pitch rates can see "what is
coming" by looking up through the canopy)

I leave it to wiser minds to organize these into a hierarchy of importance.