Why was Cain's sacrifice unacceptable?

Blood was the symbol of life. Sacrificing animals was symbolic of sacrificing our life to God.Produce from a field was important but not symbolic of life. Therefore Cain's offer was unacceptable.Those who only produce grain could have bartered or purchased a worthy sacrificial animal.

If you read very much of the Bible you'll find that blood symbolizes life. It doesn't restate that in every few verses, so one must have a little overall knowledge to clarify some of the verses.
If we were to write today about something commonly accepted, such as a microwave, in the future someone might not understand the term without an overall knowledge of our times. This happens in much of the Bible and causes a lot of misunderstanding for those who only dwell on small segments of writing.

The symbolism of giving your best or of giving your life is what giving to the Lord is all about. The 'best' of your possessions would be the unblemished. Giving the first born is placing your gift ahead of yourself. Giving of something with life, or blood, is a symbol of giving yourself to God.I understand your position and sure don't want to argue that you're wrong because none of us know completely.