The reason is simple: according to The Sacramento Bee, Newsom declined to stay within campaign limits!:

Newsom, unlike his Republican opponent John Cox, declined to accept the $14.5 million general election campaign campaign spending limit for gubernatorial candidates.

State law requires candidates for state office to keep below expenditure limits — which vary depending on the office being sought — if they want to purchase a 250-word candidate statement in the information guide that the state sends to approximately 19 million registered voters.

According to state election records, the Newsom campaign has spent $21.9 million this year as of Sept. 22. The Cox campaign, by comparison, has spent $8.9 million.