Pokemon Blue is the third version.
It was released sometime after the original Red and Green versions with updated sprites and other minor changes. Furthermore, its mascot is the third of its respective trio.
Pokemon Yellow, or rather Pocket Monsters Pikachu, was a special edition of Red, Green, and Blue that was specifically targeted towards (but not developed exclusively for) the anime fans. Yellow version also changed the available starter (into Pikachu), and made the 3 original starters available throughout the game--a one-time break in tradition which has never happened again, not even in subsequent third versions.
As the games evolved, their respective third versions began introducing more and more features exclusive to them that weren't available in the previous paired versions. Given Crystal's only changes were animated sprites, the GS Ball event and the new Cellphone features, and the subplot with Eusine and Suicune, it's perfectly understandable that Blue's changes wouldn't even measure up to Crystal's--the same way Crystal wouldn't measure up to Emerald, Emerald to Platinum, etc. It's also important to understand Crystal was also a GBC game, unlike Gold and Silver, so it's possible the GS Ball and the Cellphone features wouldn't even be exclusives if Gold and Silver had been released for the GBC.
It was released sometime after the original Red and Green versions with updated sprites and other minor changes. Furthermore, its mascot is the third of its respective trio.
Pokemon Yellow, or rather Pocket Monsters Pikachu, was a special edition of Red, Green, and Blue that was specifically targeted towards (but not developed exclusively for) the anime fans. Yellow version also changed the available starter (into Pikachu), and made the 3 original starters available throughout the game--a one-time break in tradition which has never happened again, not even in subsequent third versions.
In addition to that, I don't believe Blue's changes were enough to put it in the same category as Emerald, Crystal and Platinum.
As the games evolved, their respective third versions began introducing more and more features exclusive to them that weren't available in the previous paired versions. Given Crystal's only changes were animated sprites, the GS Ball event and the new Cellphone features, and the subplot with Eusine and Suicune, it's perfectly understandable that Blue's changes wouldn't even measure up to Crystal's--the same way Crystal wouldn't measure up to Emerald, Emerald to Platinum, etc. It's also important to understand Crystal was also a GBC game, unlike Gold and Silver, so it's possible the GS Ball and the Cellphone features wouldn't even be exclusives if Gold and Silver had been released for the GBC.