It is a general, and very negative, trend in our society atm that we tend to look back rather than forwards. Everything gets rebooted, everything gets taken "back to its roots", ignoring everything good that has been told since because "it's not the original".
The difference with
Star Trek as a franchise is that the Berman series all looked forward, with few references to TOS. Each series to follow added more distance than the last, to the point where the last (
Enterprise) reimagined/retconned, etc. so much, that it sent a clear message that the jewel of the ST crown was not so welcome.
On that note, I do not consider the TOS-themed
"In a Mirror, Darkly" evidence of TOS love, as it seemed to be the end result of an attempted ratings grab for a series that strayed as far it desired--and suffered for it. That should tell anyone that the advocates of Berman's aggressive world changing were wrong, otherwise we would be on TNG film #7 and finally seeing DS9 & Voyager films going into production.
As for the ratings, TOS had to contend with much less competition in networks or other scifi shows (wasn't it pretty much the only space drama that ran during its time?) The later ST series simply were not the big fish in the small pond anymore and especially DS9 and VOY did not only compete with other properties for viewers, but against one another for a while.
If you mean competition as in sci-fi drama no matter the timeslot, then there were other sci-fi series to compete for audiences receptive to the genre. Between '66 & ''69,
Voyage to the Bottom of the Sea, The Invaders, The Time Tunnel, Land of the Giants, and even
Lost in Space were all on the air, but if one sought serious sci-fi, TOS was the winner. Occasionally,
The Invaders produced strong stories, but it fell into a formula robbing it of greater story opportunities.