That is because  Terence Crawford never had a chance to compete with big names under Top Rank promotions which he always requested. Also, the fact that Spence is dodging Crawford because the latter thinks it's not worth his time. Spence on the other hand always got his preferred fight and make sure he will win all his title fights and that's what he is today claiming 3 Championship belts under WBC, IBF, and WBA as you have mentioned.

In Terence Crawford's fight history, the expected fight against Spence will be the biggest fight in his career. If he was able to unify all the Welterweight titles beating Spence if ever, Crawford will now begin to move up weight to extend some good credentials on his career. He has lots of targets if successfully defeated Spence in the process.