The coasties couldn’t afford to operate them. The transmissions that integrate the cruising diesels with the high-speed gas turbines, basically don’t work. To fix them, the Navy would have to cut the hulls open. Cheaper to build new ships with the improved transmissions.
The proposal for Coast Guard Cutter use would be to eliminate the gas turbine and the combining transmission, and just go with diesel power. You'd lose the top end of the DCS, but would increase range and endurance by eliminating the gas guzzling turbine while still reaching 20 knots which is what most Coast Guard Cutters do today.
The Navy LCS requirement was for a 40+ knot dash speed capability, but that isn't needed for Coast Guard use.