Why was Kayin's korban rejected? The Meshech Chohcma explains that in Sefer VaYikra the Torah prohibits bringing date honey as an offering (2:11-12). Not only is date honey not acceptable, but no fruit is acceptable as a korban. Therefore, Kayin's korban from his crops was rejected, while Hevel's offering of animals from his flock was accepted.
The Meshech Chochma offers an insightful rationale for this halacha. Items which are acceptable as sacrifices all share the common denominator of requiring an investment of human effort in their production. Wheat is turned into flour before being used for menachos or baked into lechem hapanim; grapes are processed and turned into wine before being offered as nesachim; an animal is only acceptable as a sacrifice after it is eight days old because only after eight days does it require the farmers care. When one of these items is offered, the farmer makes a sacrifice not only of the object itself, but also sacrifices the labor and effort that he invested in producing the offering. Since fruits naturally ripen on the tree without any need for human intervention, their sacrifice does not demonstrate that same dedication of human effort. Therefore, they are unacceptable as a korban.