C.E. Evans wrote:
I think the real-world reason is that the producers were just used to having a ready room on TNG, VOY, and (although it was named differently) DS9 and wanted one on ENT too.
"In-universe," I think it just be a case that it was a shipboard feature that was discontinued by the time of TOS and then reinstated by the time of TNG--in other words, Kirk was robbed...
Just another symptom of the '24th century Think' that ruined Enterprise