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.

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.