It didn't help that the first contact with Klingons occurred in the very first episode, and was about 50 years earlier than had been suggested in the other shows. Then that the first contact with them didn't trigger all the conflict we'd been led to believe.
The idea that Klingon first contact came 50 years prior to TOS was from a line in the "Day of the Dove" script, which never made it to air. It somehow found it's way into the 1993 Star Trek Chronology
, despite being contradicted on-screen by Picard's comment in "First Contact" that Klingon first contact was "centuries ago". But then Picard said that first contact was a disaster which led to decades of war which led to the decision to surveil aliens before making contact, none of which happened in "Broken Bow"...
Then subspace radio shows up before the USS Horizon leaves "The Book" at Sigma Iotia II in 2168, which happened before subspace radio existed.
I'm pretty sure the exact date is conjecture from the Chronology again. "The book" showed up as set decoration on the ECS Horizon in "Fortunate Son" - perhaps they'd already been there decades earlier and before subspace radio was invented?