Zoro and Sanji have always been the biggest threats inside the Straw Hats, outside of Luffy. The World Government knows this. The World Government also knows of Sanji's affiliation with the Vinsmokes, who contrary to what the fandom wants to believe that they aren't worth their notoriety, that means jack shit in the eyes of the author, and his intent that they are considered a force to be reckoned with in the One Piece world. Just because they couldn't stand up to a Yonko (BIG SHOCKER), doesn't negate their accomplishments and infamy throughout the World.
Whether people want to accept it or not, Sanji is gaining his bounty through World Government recognizing him as a threat, and I'm sure it is eating people alive that those words he spoke to Kuma oh so many years ago, are slowly becoming a reality.