The roaring 20's roared in an era of bees and cats possessing items that they really never should have had. Ever since, scientists have struggled to measure what is truly better: the bee's knees or the cat's pajamas. Well, Google ngram to the rescue! As you probably suspected, up until 1996 (the year of the Spice Girls' seminal classic Wannabe) being the cat's pajamas was much more favourable than being the bee's knees. Then, undoubtedly at the hands of Y2K, the world's cat pajama distribution centers began to fail, creating a vacuum that bee's knees could naturally fill. Since 2000, the world's bees may have been dying, but the bee's knees have been flourishing!
NOTE: about 14 seconds into the Wannabe video, Baby Spice steals a homeless man's hat! Girl power?