Kevin Mims makes the case that Shatner is a superb actor, and his much-parodied over-the-top style was due to a couple of factors: TVs at the time had small screens and often lousy picture and audio quality; performances had to be big because the medium was small. Also, Kirk and Spock were a duo; Kirk had to be loud to offset Spock’s stillness.

[quillette.com]:

While a lot of TV actors were trying to mimic the mush-mouthed vocal delivery of big-screen movie stars like Marlon Brando or James Dean, Shatner went in the opposite direction. He enunciated his words carefully and broke his sentences into bite-sized pieces, making each clause a separate unit of delivery. He would speed up his cadence at times, and then bring it to a near halt. Shatner’s unique speaking style has been parodied countless times. Among living actors, probably only Christopher Walken’s line delivery has generated more parodies.

Also:

It isn’t just a coincidence that names like Richard Matheson, Harlan Ellison, and Rod Serling crop up frequently in discussions of Shatner’s career. Academics frequently celebrate the work of various American literary schools–the American ex-pats of the so-called Lost Generation, the writers of the Harlem Renaissance, the Beats–but few literary salons have influenced American popular culture as profoundly as the Southern California fantasists who were all brought together by Rod Serling for his Twilight Zone series and later worked on other fantasy and sci-fi shows, including Star Trek.