The fact is that any warhead that gets launched is going to travel at low velocity, even when you have a good engine on its back--the Harbinger bomb had a Class II fighter engine strapped on the back, but it still was a slow bomb, simply because of the amount of space that was taken up by the actual warhead material.
Now try strapping anything onto a Meson bomb and sending it at any respectable speed at a juggernaut which has sufficient firepower on any side to destroy something as thinly armored as a cargo container in a split second.
The GTVA decided instead to put a whole bunch of these warheads into thickly-armored, tried-and-true Orion-class combat vessels that are designed to take a lot of crap from bombers and warships and sent one in with a huge escort and the other in from secure space to exploit a tactic which they had strong beliefs toward being effective.
