Much of Star Wars has also plateaued technologically. Or at least it has relatively plateaued, until a new, but by this time extremely rare, scientific breakthrough allows for more advancement.
Also keep in mind that even on Earth weaponry consisted of variations on swords, axes, maces, spears, lances and bows for thousands of years.
A bronze sword might have been more bendable and easier to blunt than a steel one, but essentially it's the exact same weapon.
Though the out-of-lore reason is simply that people want their Star Wars to be Star Warsy.