Heretic and Hexen have always been less popular than Doom, even now with ports like Doomsday, ZDoom, and GZDoom supporting them fully. The folks at Doomworld tried to do a community Heretic wad in the same style as the Community Chest series, but it died. (unbeknown to me ofc as I've only just popped my head up on Doom boards again after a long absence)
There's also the fact that Heretic is mainly limited to caves, castles, towns and temples in it's textures, and Hexen to similar styles too. Although I did manage to create an all-steel map in my Heretic episode :p
It's a shame really, as the gameplay can often be more complex and challenging than Doom's. Heretic in particular has the same setup as Doom (straight kill-everything-and-find-the-exit maps, not hubs and puzzles) and in my experience I don't actually need -fast to get a challenge out of Heretic wads like I do in Doom (due to the monsters generally being deadlier than those in Doom). Plus none of the weapons rust from dis-use when you pick others up (sniper wand anyone?) and the tome gives double the options. (mix of single-target and spread/AoE attacks)
Hexen's main problem IMO has always been the small number of weapons, and it's also notable that the shadowcaster cheat isn't in ZDoom/GZDoom. Hubs do exist in the archive (there are some fine ones too) but few and far between. Ichor's Fury megawad (with the tome) and my Serpent megawad (with 7 weapons per class) will change that though.