Star Trek: The Original Series introduced many of the aliens we still see today. Some have even gone on to be bigger threats than they were to Captain Kirk and his crew. The Gorn from Star Trek: Strange New Worlds comes to mind. While Kirk fought a man in a lizard suit, Captain Pike and his crew are fighting things that look like they jumped off the screen of Jurassic Park. And all of that is due to the passage of time and money.
When Star Trek first aired, it was in the 1960s, and the studios didn't have millions of dollars to use for a science fiction show...or probably any show, for that matter. So they had to use what they could get for the money which is why Klingons were more of a threat to Kirk and the Enterprise than any other alien. The Klingons just had to wear make-up and wear distinctive clothing.
But, originally, according to Judith and Garfield Reeves-Stevens, in their book, The Art of Star Trek [via August/September 1997 Star Trek Communicator, the Magazine of the Official Star Trek Fan Club], "Romulans were originally intended to be more of an ongoing threat, but the makeup requirements proved too expensive. Klingons were cheaper."
As the years advanced and the Klingon Empire and the Federation became allies, the Romulans were put forth as stronger enemies by shows like Star Trek: The Next Generation and Star Trek: Deep Space Nine. Fast forward twenty or so years after the end of The Next Generation and after the Romulan War, as shown in Star Trek: Picard, Romulans slowly integrated into Federation society, with Admiral Picard and Laris, a Romulan who was one of the caretakers of his home, even beginning a romantic relationship.
Now, with the amount of money spent per episode of a Star Trek series, the creation of enemy aliens isn't as impossible as it was for The Original Series. In fact, some could match what is brought to theaters in cinematic powerhouse movies. But The Original Series worked with what it had, and although the aliens are a bit cheesy now, they were seen in the 1960s as creative. And they are still some of our favorites!