Neither of the other two curses contains such a lengthy reason for God's pronouncement.
Why God does this probably lies in the fact that Adam was not deceived when he sinned (I Timothy 2:14). God saw the need to explain to him why he was being cursed so that Adam would not repeat the grievous error again. Events had deteriorated much too far already, and this preamble is an attempt to shore up the damage by pinpointing to Adam the cause of the problem.
God's reason for Adam's curse comes in two parts: 1) He obeyed Eve, and 2) he disobeyed God. He sinned by doing something he should not have done, as well as by not doing what he should have done! Like most sins, his was an act of commission and omission.
God first exposes Adam's abdication of his leadership role in the marriage. Though Ephesians 5:21 says to "[submit] to one another in the fear of God," Adam yielded to his wife in an area he knew God had specifically commanded them to do otherwise. He should have led, not followed.
Genesis 3 provides no explanation for why Adam followed her lead; verse 6 tersely says, "She also gave to her husband with her, and he ate." It suggests he weakly acquiesced to do her bidding with a "whatever you say, dear" attitude. He chose to appease her rather than correct her.
Then God reminds him that he had directly disobeyed His very clear command: "Of every tree of the garden you may freely eat; but of the tree of the knowledge of good and evil you shall not eat, for in the day that you eat of it you shall surely die" (Genesis 2:16-17). God's words in Genesis 3:17 cut straight to the heart of the matter. However Adam had justified to himself that he could eat the forbidden fruit, he had still committed sin in God's eyes.
As the wording suggests, the two reasons go together; it is an Old Testament pre-statement of Peter's words to the Sanhedrin in Acts 5:29: "We ought to obey God rather than men." God's commands take precedence over any words, acts, or persuasions to the contrary - even if they are delivered by a beautiful, naked woman offering to feed us fruit!