There is about a 100 year gap between ENT and TOS. As we all know, due to budget constraints the Klingons in TOS looked more human. It wasn't until Gene Roddenberry got the budget for the Motion Picture that he decided to change to the look of the Klingons, which went on as their look for TNG, DS9, and VOY. Since, the generation of Star Trek fans that had grown up with TNG, DS9, and VOY identified Klingons with the ridges, that was how they were going to look in ENT. Nevertheless, ENT will give us the reason for Klingons without ridges in TOS. The funny thing is that in DS9's episode 'Trials and Tribble-ations' Bashir and O'Brien ask Worf about why Klingons in the 23rd century had no ridges and he avoided to provide an answer, and this episode aired way before ENT was in development. That being said, Season 4 of ENT is when the writers decided to tackle a lot of establishing canon to stuff we briefly heard of from the previous series, such as giving more information about the Vulcans, clarifying the lack of Klingon ridges in TOS, and some others (avoiding not to spoil too much). ENT had the responsibility to further support canon fro things we had briefly knew about or had been mentioned in TOS, TNG, DS9, and VOY. It also has to look less technologically advanced compared to the other series as it was a prequel to what we already knew of the Star Trek Universe.
Lastly what is very interesting about ENT is that fact that it is the humans who are less technologically advanced compared to the Vulcans, Andorians, and Tellarites who is able to end the conflicts between them and convince them to forge an alliance, which would eventually lead to the Federation. It was
Ambassador Soval who told Admiral Forest that it took the Vulcans 1000s of years to achieve what human have achieved in a short amount of time.