Because surface ships have better communications with shore, and would be more likely to receive a message quickly if AEHF, MILSTAR, and TACAMO are all destroyed in a surprise attack. That's useful both for ordering a retaliatory launch, and detecting if the ship has been attacked/destroyed.

The Russians have also deployed a similar missile on surface ships, and the land based version is in violation of the INF treaty. There is a desire to show that we are willing to respond to an escalation, and to counter it without violating the treaty ourselves.