To be honest, I think it's one of those things that most fans would have been happy to accept as a "fact of life" - i.e. we all know Klingons look different in TOS because the budget was smaller, but in our minds we're happy to ignore that. Suspension of disbelief.
Unfortunately, the (great) DS9 epsiode "Trials and Tribbleations" ruined that - put Worf together in a room with TOS Klingons, and one of the characters is going to say *something*!!! So they had to come up with a canon explanation for it.
TBH, I've never watched Enterprise and never will, so I just assume they always had the bumpy foreheads. :P (and just put "Trials" out of my mind)....
.... I greatly liked the solution of DS9, by having Worf just acknowledging the differences and still
(very grumpily) refusing to explain it
I just watched those two eps of ENT for the first time (haven't seen all eps of TOS and ENT). While fine eps in their own right, I don't think such an explanation was 'necessary'.
As for real world reasons, I believe I read somewhere that, had they had the budget (and the make-up skills), they would have been portrayed in TOS as less human in appearance as well.