The climate was different. Completely different.
TNG was literally a pioneering milestone in television history: the resurrection of
Star Trek. After TNG, DS9, and VOY, the market had been there and done that three times already.
Plus, by the time of ENT, the syndication landscape was just different generally in a lot of ways, and the Internet was arriving.
What might have been forgivable for TNG just did not necessarily slide for ENT.
ETA: Plus, and most critically, all that other stuff aside, TNG's ratings did not go ever downward. After a correction at the beginning of the third season, when ratings actually were finally in the process of sagging after the first two years, the producers managed to get the ratings of TNG to go
up [
http://www.madmind.de/2009/05/02/all-star-trek-movies-and-episodes-in-two-charts/]. Such a turnaround never happened for ENT, and the ratings trend for ENT was just a continuation of that for DS9 and VOY: ever downward.