>>>What is your opinion on why Cain's sacrifice was not acceptable>>>
You need to look at the further lesson in Gen 3 which was God's Personal instruction on acceptable sin offerings
which He showed Adam.
For me it explained the difference in God's acceptance of Cain and Abel's offerings.
consider Gen 3:21, where God made coats of skin as a 'kaphar' for Adam & Eve's sin.
It does not take too much
thinking to realize those animals could not have donated their skins without losing both their lives and their blood.
also realize that this was the first DEATH Adam had ever witnesed. Here was some of his little animal buddies giving their
life for his sin.
Can you see the vicarious sacrifice prophecy of Jesus yet?
I can imagine it made a big impact
on Adam. So much that He surely told the boys what God expected. For me, that is sufficient information to know why Abel knew
what was an acceptable sacrifice, and also how to do it.
If Cain had not known there would be no transgression, for
it is written:
41 Jesus said unto them, If ye were blind, ye should have no
sin: but now ye say, We see; therefore your sin remaineth.(KJV)
Hence, Cain also certainly knew what God required,
but either had no animals, or didn't want to part with them. He just wanted to do it his way, just like his offspring does
There is nothing new under the sun...