Why didn't they officialize the Enterprise to be a functional time travel machine as of season 1 episode 19 of TOS?

Star Trek TOS "Assignment Earth":

Captain's log. Using the lightspeed breakaway factor, the Enterprise has moved back through time to the twentieth century. We are now in extended orbit around Earth, using our ship's deflector shields to remain unobserved. Our mission, historical research. We are monitoring Earth communications to find out how our planet survived desperate problems in the year 1968.

http://www.chakoteya.net/StarTrek/55.htm[1]

Star trek IV: The Voyage Home:

SPOCK: There is one possibility, but I cannot guarantee its success. We could attempt to find some humpback whales.

McCOY: You just said there aren't any, except on Earth of the past.

SPOCK: Yes Doctor, that's exactly what I said.

McCOY: Well, in that case... Now wait just a damn minute!

KIRK: Spock, start your computations for time warp. ...Bones, you come with me.

And:

McCOY: Are you really going to try this time travel in this rust bucket?

KIRK: We've done it before.

McCOY: Sure, slingshot around the sun. If you pick up enough speed you're in time warp. If you don't, you fry.

KIRK: Would you prefer to do nothing?

McCOY: I'd prefer a dose of common sense. You are proposing to head backwards in time, find humpback whales, then bring them forward in time, drop them off, and hope to hell they tell this Probe what to do with itself!

KIRK: That's the general idea.

McCOY: That's crazy!

KIRK: You have a better idea, now's the time.

http://www.chakoteya.net/movies/movie4.html[2]

And a different time travel process was accidentially discovered at the end of "The Naked Time":

KIRK: Checked elapsed time, Mister Sulu.

SULU: My chronometer's running backwards, sir.

KIRK: Time warp. We're going backward in time. Helm, begin reversing power. Slowly.

SULU: Helm answering, sir. Power reversing.

SPOCK: We're back to normal time, Captain.

And:

KIRK: The time warp. What did it do to us?

SPOCK: We've regressed in time seventy one hours. It is now three days ago, Captain. We have three days to live over again.

KIRK: Not those last three days.

SPOCK: This does open some intriguing prospects, Captain. Since the formula worked, we can go back in time, to any planet, any era.

KIRK: We may risk it someday, Mister Spock. Resume course to our next destination, Mister Sulu.

http://www.chakoteya.net/StarTrek/7.htm[3]

"Assignment Earth" is unclear whether they use the method from "The Naked Time" or the method from "Tomorrow is Yesterday", but Star trek IV: The Voyage Home clearly used the method from "Tomorrow is Yesterday".