God is holy. In heaven exists a Being who decides whether or not we take another breath. This holy God deserves excellence, the very best we have. From the beginning of time, He has been clear that some offerings are acceptable to Him and others are not.
5 but on Cain and his offering he did not look with favor. So Cain was very angry, and his face was downcast.
When I fed them, they were satisfied;
when they were satisfied, they became proud;
then they forgot me.
8 But when you present the blind for sacrifice, is it not evil? And when you present the lame and sick, is it not evil? Why not offer it to your governor? Would he be pleased with you? Or would he receive you kindly?” says the LORD of hosts.
10 “Oh that there were one among you who would shut the gates, that you might not uselessly kindle fire on My altar! I am not pleased with you,” says the LORD of hosts, “nor will I accept an offering from you.