Because he was 'rich, handsome,' she tells Nelly herself. His social standing appealed to Catherine because it would mean her married life would be a luxurious one. Yet, she tried to convince herself that she wasn't doing it for selfish means but for Heathcliff. She wanted Heathcliff to 'rise' in society and not be the mere servant that Hindley had reduced him to. Of course she didn't want to give Heathcliff up after marriage either...

Alot of ppl say she only loved Heathcliff but I don't think its true. She simply loved Heathcliff MORE and with more intensity. She said herself that her 'love' for Edgar was like the 'foliage in the woods. Time woould change it...' so she felt something for Edgar but not enough to justify the marriage on love

"I mean, if you love someone so much you would do anything to be with them, right?"
Not necessarily, if you knew that you would likely be unhappy with them or marrying them would severely diminish your quality of life. Cathy might have been ostracized if she had married Heathcliff.
Not to mention the fact that she and Heathcliff's relationship wasn't at its best when she made her decision. She has become so aware of Heathcliff's faults at the time and then there's Edgar who's nice and good-hearted and proposes first. Common sense says she should marry him and I think their marriage was actually pretty good, all things considered. (She did love both of them.)
But I think she wanted to be with Heathcliff and might have turned Edgar down had Heathcliff not disappeared into the night and led everyone to believe they might never see him again.