Because the serpent was shrewd. He knew that Adam had utterly failed because from the BEGINNING man has substituted what G-d has said for man's tradition esteeming them better. Notice what Eve says to the serpent. Gen 3:
2 The woman said to the serpent, “From the fruit of the trees of the garden we may eat;
3 but from the fruit of the tree which is in the middle of the garden, God has said, ‘You shall not eat from it
or touch it, or you will die.’”
Gen
That is NOT what G-d said to Adam. Gen 2:
16 The LORD God commanded the man, saying, “From any tree of the garden you may eat freely;
17 but from the tree of the knowledge of good and evil
you shall not eat, for in the day that you eat from it you will surely die.”
The moment Eve told the serpent that she wasn't allowed to touch it, he knew he had her. G-d spoke directly to Adam concerning what could and could not be eaten in the garden NOT Eve.
Furthermore, it is HIGHLY unlikely that Adam was with Even when this occurred. The enemy attacks when you are vulnerable to attack. Adam would have never allowed Eve to eat the fruit because Adam had firsthand knowledge.
In my personal opinion, Adam and Eve had most likely been in the garden for a while we get hints of this by what happens shortly after they have eaten...
8 They heard the sound of the LORD G-d walking in the garden in the cool of the day..... This suggests that they were accustomed to when G-d would come during the cool of the day. We don't know how long Adam was in the garden before Eve was created. Nor do we know how long they were in the garden before she was deceived and gave fruit to Adam to eat.
The bigger question is why did Adam eat?
IMO Adam loved eve and knew if he did not eat that she would be left alone because of the sin of disobedience. It is key to remember that prior to their sin Adam and Eve were PERFECT. They knew no sin so Eve's love for Adam would have been perfect. Imagine that you companion was beyond anything you could ever dream of and how they treated you made you feel completed. Adam KNEW what it was like to be alone and he knew what it was like to be with her. Adam probably could not fathom losing her. You see unlike every other love story or marriage Eve was literally taken from Adam. There could not have been a more perfect match for Adam than Eve. G-d called creation when he rested "Very good" I have seen vista's so beautiful they just take your breath away in their beauty. Adam and Eve would have been like this.
On some level one can understand Adam's reply to G-d when asked why he ate what was forbidden. Adam had to choose being alone again or sinning to be with Eve so, he predictably blames G-d by saying "The woman YOU gave me....." Oh that Adam had trusted G-d and had told Eve "No" We will wait on the Lord and He will fix this.... but, that was never to happen.