Because the MDA is looking at ground basing, where the interceptor does not face the naval prohibition against hypergolic (liquid propellants that ignite spontaneously in each others presence) propellants, they are looking to integrate liquid propellant into the next generation kill vehicle. (paid subscription required)

Because the motor is completely throttlable, it allows for extended range, since the vehicle can coast and reignite the motor for terminal maneuvers, and it can perform more effectively at shorter ranges, because it can have a higher thrust for a shorter flight.

The downside is cost, complexity, and issues with storing the fueled missiles and propellants.