Grace may be irresistible. But that doesnt mean its obtained automatically.
Just because you see a beautiful woman who u might find irresistible, doesnt mean that you can automatically obtain her.
You might find her irresistible. But then shortly after, learn that she has 2 young children as well. So then you say to yourself "Ahhh i REALLY want her....but...i cant be bothered taking care of her children as well. Forget that."
And so its the same way with mankind and the Lords grace. Some may desire to be with Him. But, then they learn about the things they will have to sacrifice / put away, in order to obtain His favor. Then they say to themselves "No i cant give up my ways, and the things i treasure dearly in this world! Grace is certainly appealing...but Forget that!"