Question

it is not so much the language

it is not ___ much the languages as the background that makes the book difficult to understand
Athat Bas Cso Dvery
是not so（as）+原级+as “不及”,但为什么不能用as?

Answer 1

not so much.as 的意思是 与其说……不如说
not as much.as 意思是 不及 如果中间是clever,那就是不及……聪明

Answer 2

与其说是语言倒不如说是背景让这本书难以理解。so much as意思是与其……不如……