Because the phrase, "should not" is Late Middle English and past tense for "shall not".
Also, notice that the entire phrase is past tense, as in "already done" / "it is finished".
We find further support here, SBG, only in the present tense:
" He that believeth on the Son hath everlasting life: and he that believeth not the Son shall not see life; but the wrath of God abideth on him." ( John 3:36 ).
" Verily, verily, I say unto you, He that heareth my word, and believeth on him that sent me, hath everlasting life, and shall not come into condemnation; but is passed from death unto life." ( John 5:24 ).
All of these are present tense and say that the person that "hears" His word and believes on the Son, has ( not "will have" ) everlasting life.
He that believes not shall not see life, but the wrath of God abides ( present tense ) on him.
He that believes shall not come into condemnation, but is passed from death to life.
Salvation is a certain thing for those who are Christ's.