Because they are not fans of Trek. They do not understand why people would like DS9 because they never watched it.
The season’s arc was written by Bryan Fuller, who wrote for DS9 and VOY. (Fuller left production, but he’s still credited for the story on the first 3 episodes.) And there’s still plenty of fans working on Discovery. Kirsten Beyer wrote Voyager books before writing for it; Jonathan Frakes will supposedly direct an episode. And I’m sure most of the writers have seen lots of Trek.
Also, the idea that good Trek can only be made by fans of the old shows is silly. Bragga and Berman were not big fans of TOS and TNG is good. And Roberto Orci is a huge fan of TOS and I don’t think Into Darkness is very good or understands TOS very well. There is also plenty of room for a Star Trek show to be different from the hundreds of hours of existing Star Trek.
I liked the 3rd episode alright and I’m interested to see where it goes. The tone of it doesn’t really match with just 10 years before TOS, but this is more in the “responding to the present” mode of Star Trek, rather than then “socialist utopia” mode that TNG often did.