If I’m remembering correctly, all Jedi lightsabers in the Prequels were either blue or green. The only exception was Mace Windu’s purple saber. Fans and EU writers later ascribed meaning to these colors, with blue and green denoting positive things like “peace” or “life”. Apart from Star Wars, psychologically speaking, blue and green are associated with feelings of serenity, calm, life, nature, etc - things that reflect the Jedi philosophy.
The Sith used red lightsabers - a color associated with violence, passion, fire or blood. Windu’s purple saber apparently reflected his ability to partially tap into the Dark Side in limited ways - thus his lightsaber is purple, a combination of blue and red. (In reality, I think Sam Jackson just thought it would be cool to have a purple lightsaber. Fans or EU writers derived the lore after the fact.)
Perhaps yellow never appeared in the Prequels because colors like yellow and orange share certain psychological similarities with red, in the sense that they invoke things like “heat”, “fire”, “alert”, “warning”, etc., and thus seem a bit too “Sith-y” perhaps, or at least not sufficiently “serene” to be appropriate for a Jedi. I don’t claim that George Lucas consciously went through such a thought process - it’s just my conjecture.
Later on, Disney probably started depicting yellow lightsabers merely for the novelty factor. They wanted to show some cool new lightsaber colors we hadn’t yet seen, and there simply aren’t that many colors remaining that would work visually. So they went with yellow.
