Because John 6 says they cannot and Romans 3 says that they do not. (It really was not that complicated.)
Thats why I said or I believe I said, when scripture says they do not it equates to they cannot.
The unregenerate sinner will not come to Christ Jn 5:38-40
38 And ye have not his word abiding in you: for whom he hath sent, him ye believe not.
39 Search the scriptures; for in them ye think ye have eternal life: and they are they which testify of me.
40 And
ye will not come to me, that ye might have life.
because Jesus went on to say they cannot come Jn 6:44,65
44
No man can come to me, except the Father which hath sent me draw him: and I will raise him up at the last day.
65 And he said,
Therefore said I unto you, that no man can come unto me, except it were given unto him of my Father.
So our unwillingness to believe in Christ by nature is due to our inability to believe.
And understand, to believe in Christ, the True Christ isnt believing in the false christ that died for everyone in the world to make salvation possible if you only do this or that, but in that Christ who specifically came to die for and save His Sheep from their sins, and did accomplish that. Matt 1:21
21 And she shall bring forth a son, and thou shalt
call his name Jesus: for he shall save his people from their sins.
Jn 10:11,15
11 I am the good shepherd: the good shepherd giveth his life for the sheep.
15 As the Father knoweth me, even so know I the Father: and I lay down my life for the sheep.
Jn 19:30
When Jesus therefore had received the vinegar, he said, It is
finished: and he bowed his head, and gave up the ghost.
Now all His Sheep that He died in the stead of, are legally saved from their sins, period ! Its finished, and in their lifetime the Spirit is dispatched to apply their Salvation in the New Birth, and then lead them into the Truth of the Gospel of their Salvation !