Why did Catherine marry Edgar ?

Edgar is the logical choice for one to marry Catherine because he is of a more appropriate social class in contrast to Heathcliff who is considered "low." If she loves him, it is only because he is kind to her and she has spent some time in his company. At any rate, her love for Heathcliff is much more passionate and intense than any feeling she might have for Edgar.

because his Social status and his luxury life,she saw that when she stay at Thrushcross grange for some time ,and saw a comfortable and beautiful life in this house ,and if she marries Heathicliff she will degrade by the people and society, and the suitable suitor to her in her opinion is Edgar she want to peace of her mind for her entire life.