Because it was never the point, never really written into the show from the word go and only really was ever considered when various showrunners realised that fans were expecting it.
Trek is about a message, about ideas, the setting is just that, a setting which allows those ideas to be expressed. It was never supposed to be a Tolkien esque epic world to be explored, which is exactly why it falls apart so readily under analysis.
TOS was definitely not about continuity. I mean, outside of the characters on the Enterprise, you had what?
- Mudd reappearing once
- Arguably reusing The Cage footage for The Menagie
- A reference to the Treaty of Organia in The Trouble With Tribbles
- The Klingons and Romulans used as recurring antagonists (albeit with different characters each time)
That's it really. The show was entirely episodic, and (other than the Menagerie two-parter) no episodes ever had sequels. There weren't even off-hand remarks inserted into the script about past adventures. This changed quickly though after TOS. TAS had several episodes (Yesteryear, More Tribbles, More Troubles, Mudd's Passion) which were basically fankwanky sequels to threads from TOS.
What set things in motion, however I think was TWOK. TMP was an attempt at a new adventure, and didn't work. TWOK was a direct sequel to a TOS episode, and acclaimed. It was followed up with two further movies which gave the TOS characters their first true serialized arc. In general with Trek movies, the fanwankier the movie, the more popular they were. TUC, for example, while it's not a direct sequel, seems to have been in part written to help square the circle of how the Klingons evolved from being ruthless antagonists to the uneasy allies we see in early TNG - serving as a "prequel" to the TNG era. And the most beloved TNG-era movie - First Contact - is double fanwank, fleshing out Zephram Cochrane and the origin of human warp flight and serving as a sequel to BoBW. In contrast, the other Trek movies - which were jumped-up TNG episodes - were mostly not popular. Hell, even the reboot movies follow this format, even if not set in the prime continuity. The first two movies are full of fankwank like Pike and Khan. Star Trek: Beyond is a standalone adventure, and underperformemed.
We could also extend this to series post TOS. TNG's first-season attempts to bring back stories from TOS were lill-advised, but people generally liked the episodes which built on TNG's developing cast of recurring characters. DS9 broke from the Trek format a bit, but it was the fanwankiest of the series by far in terms of expanding on what TOS and TNG had established. VOY went its own way, and never did as well in ratings and critical acclaim. ENT floundered early on when it basically took the VOY format, but as it developed its own mythos in season 3, and turned to pure fanwank in season 4, it won over more hardcore fans - if not general viewers.
The fact of the matter is the bulk of Trek fans don't want the TOS format updated for today. They don't want to use the Trek formula to endlessly explore new sci-fi issues. They want a deepening exploration of what's already been established.