Estella married Drummle because she knew he was a terrible person and everyone would be perplexed and hurt that she married him. She was more interested in hurting her suitors, because there were plenty of excellent men that wanted her. She married Drummle to spite them all.
Pip hates Drummle, because he knows that Drummle is an idiot and a brute. Estella explains to Pip that she has no heart, as far as emotions are concerned, and therefore she cannot feel.
“You must know,” said Estella, condescending to me as a brilliant and beautiful woman might, “that I have no heart—if that has anything to do with my memory.” (Chapter XXIX, p. 162)
Estella is not going to fall in love and marry. Her marriage was a cold, calculated decision to injure as many people as possible. Miss Havisham was as horrified as Pip when Estella decided to marry Drummle.
As to leading me into what you call this fatal step, Miss Havisham would have had me wait, and not marry yet; but I am tired of the life I have led, which has very few charms for me, and I am willing enough to change it. (Chapter XLIV, p. 254)
The decision to marry Drummle was the one thing Estella did have control over in her life, and it was her way of getting back at Miss Havisham for the way she was raised. She was raised to torture men, so she chose to marry a man who would torture her.
They got married because they have some of the same qualities (figure out which ones). Also She did not marry Pip because she never really liked him and her true feelings were only said when they first met and were playing together as children at the Satis House. She pretended she wanted to marry him so that Miss Havisham would be happy and so that she could get her revenge on all men.