From everything I've seen, it was mostly a combination of maintenance/complexity, cost, and the F-15 just not really needing thrust vectoring in the first place.
F-15C+ Super Eagle: https://orig00.deviantart.net/c32e/f/2018/204/d/f/f_15c__super_eagle_by_vexiphile-dci33cz.png An upgraded F-15C equipped with an AESA radar, IRST, 3D thrust vectoring nozzles based on the F-15B ACTIVE's and NF-16D VISTA's, as well as other electronic goodies carried by the F-15SA.
Marathon-class Heavy Cruiser: https://orig00.deviantart.net/0cc6/f/2018/202/0/3/marathon_class_heavy_cruiers_by_vexiphile-dchwxsj.png Epoch-class Heavy Carrier: https://orig00.deviantart.net/baaa/f/2018/202/0/d/epoch_class_heavy_carrier_by_vexiphile-dchwxk8.png Probably my favorite UNSC design alon...