Magic players are an opinionated group, and everyone plays favorites - we have favorite cards, colors, and even creature types. And every creature type has its fans, no matter how obscure, who hope for cool new examples of that type and even legendaries in the next set.
It's in R&D's best interest to have diverse legends now that Commander is a sanctioned format, but there are still many, many creature types without a legend to call their own. Mark Rosewater addressed this recently on his Tumblr question and answer blog.
When fezzhead asked him, "I was surprised to discover that there aren't any legendary Kor creatures in Magic. Any chance we might see one the next time the Kor show up?" Rosewater punnily replied:
Have we be rotten to the Kor? : )
I assume one day, we’ll make a legendary Kor.
Since white doesn't have a characteristic creature type per se yet and has to fall back on generic Humans, it's distinctly possible that the Kor will be seen much more often than they previously have. And, of course, with the high likelihood of the Return to Zendikar block, we'll surely see a Kor resister legend continuing the desperate war against the Eldrazi.
And when orlouge82 asked, "Why are there no Legendary wurms? The creature type has been a part of Magic from the beginning," Mark said:
I’m sure we’ll make one eventually.
Essentially, Wurms have the opposite problem that Kor do - Kor have yet to be made white's characteristic type; Wurms have been displaced as green's iconic type by Hydras, because it's hard to sell a legless, wingless Dragon when red has real Dragons as its iconic. Still, it's comforting to know Wurms are still very much something R&D has confidence in.