Madame Loisel marries a clerk because she is not from a wealthy family and therefore cannot marry a wealthy man.

She had no dowry, no expectations, no way of being known, understood, loved, married by any rich and distinguished man; so she let herself be married to a little clerk of the Ministry of Public Instruction. (p. 1)

Madame Loisel is beautiful, and for some reason feels that she deserves better.  Even though she does not come from a wealthy family, she feels that she deserves to be wealthy and to have the luxuries wealth provides.  So she resents her husband and her household, because it is not want she wants.  This ultimately leads to her downfall, when she borrows the necklace and lets loose a series of unfortunate events that ends with her losing her youth and beauty as well

Why did Madame Loisel not get married to a rich man?

The text tells us that Madame Loisel married Monsieur Loisel, a clerk at the Ministry of Education. Certainly, he was a man of modest means; yet, Monsieur Loisel loved his wife very much and endeavored to make her happy whenever he could.

The main reason Madame Loisel did not marry a rich man was because she did not have a dowry ("marriage portion") and was herself born into a family of modest means. Madame Loisel's background provided her little opportunity to become acquainted with men of wealth and distinction. Basically, her lack of social status made it impossible for her to socialize or fraternize with members of the upper class.

Certainly, Madame Loisel accustomed herself to her humble lifestyle; however, she was deeply unhappy with it. She felt herself born for every "delicacy and luxury." Madame Loisel's sullen discontent with the circumstances of her life precipitated her later suffering in the story.