Why do they keep going back to Kirk era?
1) Brand recognition. Kirk and Spock are iconic characters that have become synonymous with Star Trek. They have broad public appeal which means it is easier to market the movies and shows to the general public. I think a big reason why the Marvel and DC movies do so well is name recognition. Everybody knows Captain America, Wonder Woman or Superman. You can take superheroes with huge public appeal and put them in a cool looking movie and it's instant box office gold. Nowadays, with the huge competition from big franchises, name recognition becomes even more important. You are simply not going to attract a big enough audience if you lack name recognition.
2) Writer fatigue. I think writers were getting tired by the end of Voyager. They had forgotten how to write compelling character stories and were just relying on technobabble to save the day. Voyager's bat-armor anyone? So, I think TPTB wanted to go back to a more primitive era in order to get away from the uber tech of the post-Voyager era.
3) The aesthetic and visuals were looking "old fashioned". Compare the JJ Trek movies to Nemesis and you can see the huge difference in visual style. Nemesis looks like a made for TV movie whereas JJ's Star Trek looks epic and modern. Let's face it: Nemesis was a box office bomb back when it came out, it would totally bomb now in the face of movies like Wonder Woman and Guardians of the Galaxy. TPTB saw the writing on the wall and realized they needed a big refresh. And yes, TPTB could have just refreshed the Voyager look to look more modern, but I think they preferred to go back to the Kirk era since they could combine a fresh look with iconic characters. 2 for 1!
Having said all that, I personally love the post-Voyager era in terms of tech and I so wish we could get a movie or TV show set in that era. I think with good writers and good producers, it would be possible to make it fresh again. But I can understand why they go with the Kirk era, although at this point, it is starting to get repetitive. There is more to the Star Trek universe than what Kirk and Spock did!