The Stig wrote:
There will be no 'smooth-headed' Klingons because the Enterprise episode arc that 'explained' the difference was stupid and pointless. Worf's joke in 'Trials and Tribble-ations' was explanation enough.
Even in Trek, where continuity is important in varying degrees to fans, some changes just gotta be accepted as we-did-it-because-we-could-finally-improve-production-values. I don't think we're actually owed detailed in-universe explanations for those things short of "it's a long story." Certainly not entire story arcs like in ENT. That's one step away from a series of stories proving
how the TMP Enterprise is a refit of the TOS Enterprise, rather than just accepting that it's a completely different ship with the simple in-story explanation, "A refit. Oh. OK, I'll go along with that."
I've always liked two very simple in-universe explanations for the ridges.
-- The first doesn't explain the ridges, but says they were always there. However, some Klingons carry genes that are somehow triggered to occasionally produce a generation of smooth-headed Klingons among certain couples. It is a natural mutation, and occurs infrequently. The time of TOS was one of those times. The gene is dormant in most offspring, and they have ridged children.
-- The second says there were historically two races of Klingons, smooth-headed and ridged, but the smooth-headed Klingons were so small in number that interbreeding with ridged Klingons finally led to there being few if any totally smooth-headed Klingons left. By the 24th century, all Klingons have ridges of some sort.
Edited to add: These explanations don't explain the ridges on Koloth, Kang, and Kor in DS9, but those could be explained by cosmetic surgery (Klingon versions of tattoos) or a gene kicked in late in their lives because they came from smooth-headed stock. Or, they genetically enhanced themselves. Whatever. I guess I'm just saying the simplest explanations may work the best. They are the only three Klingons on screen whose contradictory appearances needed explaining, anyway.