Several reasons why some people don't believe on Christ or His Gospel !
One reason why people don't believe on Christ or His Gospel is simply because they are not of His Sheep Jn 10:26
But ye believe not, because ye are not of my sheep, as I said unto you.
Hence, any reasonable soul should see that this statement by Christ indicates that if a person would have been one of His Sheep, that He died for Jn 10:11,15, they shall believe in Him !
A second reason why people don't believe in Christ is because they weren't ordained to eternal life Acts 13:48
And when the Gentiles heard this, they were glad, and glorified the word of the Lord: and as many as were ordained to eternal life believed.
The word ordained does mean also predestined to eternal life !
And when the Gentiles heard this, they rejoiced and glorified (praised and gave thanks for) the Word of God; and as many as were destined (appointed and ordained) to eternal life believed (adhered to, trusted in, and relied on Jesus as the Christ and their Savior). AMPC
Understand something, believing in Christ is the consequence of having been ordained, appointed to eternal life !
This speaks to election. Notice the phrase " as many as" ordained to eternal life. This again denotes election or a limited amount, for if when this phrase is used "as many as" it means all without exception, then the words would have no meaning ! Like again here Jn 17:2
As thou hast given him power over all flesh, that he should give eternal life to as many as thou hast given him.
Once again, if "as many as" means all without exception, the words are meaningless, but if restricted, it speaks once again to limited election !