ONLY because the KJVO made it so!
Actually it goes back to a debate between Westcott And Hort (W&H) versus John Burgon (JB).
This debate is contained (for the most part) in the Book The Revision Revised John Burgon, 1871.
There are others...
Free.
The Revision Revised by John William Burgon
Basically W&H felt that older is better (mss B and Aleph) shorter is better in that they felt the Byzantine priests/scribes embellished and conflated the "Traditional Text" (as Burgon called it) to smooth it out.
JB had his own schema of authenticity.The Seven Tests of Truth found in The Traditional Text of the Holy Gospels.
This present book is another of Dean John William Burgon's masterpieces. It is loaded, as are all of his books, with overwhelming evidence from manuscripts, lectionaries, ancient versions, and church fathers. Dean Burgon does three things in this volume: (1) He outlines his seven tests of Scriptural truth; (2) He proves the superiority of the traditional received text; and (3) He shows the inferiority of Westcott and Hort's favorite manuscripts-Vatican and Sinai, that is, B and Aleph. His arguments are powerful and convincing!
The Traditional Text of the Holy Gospels