Pretty much what @
Blackpoiint said. Cave horrors are more of a medium task despite the 58 slayer requirement to kill them. Making them a hard task will probably make that area more crowded. By the time players reach the requirements for hard slayer tasks, chances are they should have a slayer helm made by now, so there would be no point of killing cave horrors for more black masks as an ironman.
No support as well just as is its already a money maker for people with guthans making it a hard task would make it even more crowded making it harder for newer players (mostly irons) to obtain a black mask the only way to offset it would be to increase the spawns, which would in turn devalue black masks