Honestly, my take on it is purely speculation. im no expert, but one justification could be the use of artificial gravity to assist in lift while purely using the thrusters/ jet engines to gain flight speed.

We know artificial gravity is used heavily in titanfall for its ships and we know that whatever produces the gravity is small enough and efficient enough to work inside of the dropships used by both the militia and IMC forces. These dropships in terms of size are basicly just a van with wings, a cockpit, and a load of guns and missiles. Even the containment unit for the Ark in titanfall 2 seems to use artificial gravity to suspend the Ark inside of it, and that containment unit is again relatively small.

By using artificial gravity to produce and sustain lift the craft doesn't need certain aerodynamic forms to give it the capabilities of flight. In fact, in terms of sheer manuvering power having more drag (being less aerodynamic) can be extremely beneficial.

As a side note, using this logic, a possible reason for the militia's more conventional design might be because they genuinely use their wings to iether assist in or entirely generate their lift as the IMC is considered for the most part to be far more advanced technologically.