Heat and damage.
STK-3F has twin LRM-10s (.6 damage each at long/medium range, 1.2 total). 1+ is normally enough to get LRM. However, the STK-F has major heat issues, and this lowers the LRM damage to less than 1, so it doesn't get the special. Similar with SRM-6s. Each is 0.8 at medium range, for 1.6 total. And would qualify for SRM based on that. But the huge heat problems reduce the SRM damage to less than 1.
STK-3H drops the large lasers, and much of the heat problem. It also has LRM-20s instead of LRM-10s, though it also has low ammo that negate some of that advantage. But it qualifies for LRM and SRM because in large part because of the better heat efficiency.