This question troubled me because I could not remember Pip marrying Estella. It caused me to bring out the text.
Keep in mind that Dickens wrote this as a serial- a bit like today's soap opera- published periodically over more than a year. In the original, it was not possible for Pip to marry Estella, because they parted with the understanding that this was the end. In later editions, parts were changed and an afterword alluded to a marriage.
As to why he didn't marry Biddy, again keep in mind that it was a serial. In the twists and turns of Pip's life it was not meant to be.
As we read Dickens, we find that many times the serial format causes plot twists that are unusual for scripts, but often realistic in life.
I would also add, that Pip simply idealized Estella since the beginning. He had her as some sort of goal in his life and she symbolized all the gentlemany and finer things in life since she came from a better upbringing that he. The fact that she was so shallow may have instilled in him an interest to "conquer her". On and all, he simply could have not ended up with Biddy because he simply could not love her.
In Ch.39 Magwitch returns to England and shocks Pip by claiming that he is his benefactor and asserts, "Look'ee here, Pip. I'm your second father. You're my son more to me nor any son."
In Ch.50 Pip makes the startling discovery that Magwitch is Estella's father: "and the man we have in hiding down the river, is Estella's father."
This would make Pip and Estella almost brother and sister, and for Pip to marry Estella would be incestuous - something which the contemporary Victorian readers would have found impossible to accept. And that is why in all the standard editions of the novel Pip and Estella do not marry at the end of the novel and both of them part as friends:
"We are friends," said I, rising and bending over her, as she rose from the bench.
"And will continue friends apart," said Estella.
It should also be remembered that Dickens' friend, the novelist Edward Bulwer Lytton pleaded with him to unite Pip and Estella, and after much persuasion on his part Dickens did so. This version of the ending in which Pip and Estella are united in marriage was used only once by one maverick editor, namely, George Bernard Shaw in his 1937 edition of the novel.
However,most importantly in Ch.17 we find Pip struggling with his feelings concerning both Biddy and Estella. Pip confesses to Biddy that he wants to become a gentleman and when Biddy asks him, "do you want to be a gentleman, to spite her or to gain her over?" Pip is confused and replies "I don't know." This is because he has become foolishly infatuated with Estella and is unable to free himself from her power even though she is always rude to him and torments him both physically and emotionally. Pip himself admits that, "Biddy was immeasurably better than Estella" but is unable to decide to marry her or not because of his infatuation for Estella, and Pip confesses and acknowledges this fact: "All the while knowing the madness of my heart to be so very mad and misplaced." Finally, he pleads with Biddy in the follwing manner:
"Biddy ... I wish you could put me right ... If I could only get myself to fall in love with you ... that would be the thing for me." To which Biddy replies very sagely, "But you never will, you see."
Pip in Great Expectation actually never married Estella , but he definitely did idolize her . Pip did not feel attracted to Biddy because he was attracted to a female's physical appearances . Although Estella was a very harsh and heartless character , Pip stayed " in love " with her throughout the novel. Estella to him was a very beautiful woman , also Estella belonged to the upper class. Pip himself has always wanted to be a " gentleman " which to him meant that you have to rich and successful . Probably being with Estella, Pip would feel more like a " gentleman " as well .
he never married no one and biidy is married to joe
Just to tell you, Pip never married Estella. This is indeed a sad thing, because earlier in the chapters, this was one of Pip's expectations to marry Estella, but he never got to. This is all of Miss Havisham's plan to make Pip suffer, because she suffered the pain of "love" too. Now, Estella is taught by Miss Havisham, treating Pip cold and harsh, saying that she won't marry him, and she doesn't love him anymore. Estella ends up marrying Drummle.