A bunch of reasons! I think from the evidence it's pretty clear Lizzie *couldn't* have done it (just time wise: the window for Andrew Borden's death is SO small, and she had no blood on her shortly after). BUT, I do think that her behavior suggests she knew more than she should have. And there are several things that I can't get away from, that make it impossible for me to believe this wasn't a family affair:
1) Everyone in that house expected to die. Abby thought they were being poisoned (rather than the fairly obvious assumption that it was just food poisoning). Lizzie told a close friend she had a terrible feeling that something awful was going to happen.
2) The house itself--the physical house as well as the family--was just bizarre. Half the doors were locked, such that you had to go downstairs and physically leave the house to move from one second-floor room to the next. On its own, not such a big deal. Added to the weirdness already in that family, I think it's suggestive of a lot of fear.
3) Emma (the older sister) was out of the house staying with friends at the time of the murder--the first time she had stayed overnight away from home in years. I can't believe that's a coincidence.
4) Why were both daughters unmarried and still living at home in their thirties/forties? Obviously, some people just don't find anyone, but they were well-connected, well-off, reasonably attractive women. I suspect their father wanted them to stay at home (a controlling personality seems in keeping with the evidence on his character).
5) Both the mother and the father died, and in a horribly brutal manner. If it was only one or the other, or if the death was of a less violent nature, I could believe it was an outside, or it was a money matter, or something like that. The fact that it was both of Lizzie's parents makes me think it was someone in the family, and it's well established that when someone is murdered in a particularly violent fashion, 9-10 times out of 10 the murderer was very close to them.
tl;dr: weird family clearly had secrets, death was so violent it was clearly personal. Marcia Carlisle has an essay explaining this theory much better than I can here: http://www.americanheritage.com/content/what-made-lizzie-borden-kill?page=show
Re: Conspiracy theories
1) Everyone in that house expected to die. Abby thought they were being poisoned (rather than the fairly obvious assumption that it was just food poisoning). Lizzie told a close friend she had a terrible feeling that something awful was going to happen.
2) The house itself--the physical house as well as the family--was just bizarre. Half the doors were locked, such that you had to go downstairs and physically leave the house to move from one second-floor room to the next. On its own, not such a big deal. Added to the weirdness already in that family, I think it's suggestive of a lot of fear.
3) Emma (the older sister) was out of the house staying with friends at the time of the murder--the first time she had stayed overnight away from home in years. I can't believe that's a coincidence.
4) Why were both daughters unmarried and still living at home in their thirties/forties? Obviously, some people just don't find anyone, but they were well-connected, well-off, reasonably attractive women. I suspect their father wanted them to stay at home (a controlling personality seems in keeping with the evidence on his character).
5) Both the mother and the father died, and in a horribly brutal manner. If it was only one or the other, or if the death was of a less violent nature, I could believe it was an outside, or it was a money matter, or something like that. The fact that it was both of Lizzie's parents makes me think it was someone in the family, and it's well established that when someone is murdered in a particularly violent fashion, 9-10 times out of 10 the murderer was very close to them.
tl;dr: weird family clearly had secrets, death was so violent it was clearly personal. Marcia Carlisle has an essay explaining this theory much better than I can here: http://www.americanheritage.com/content/what-made-lizzie-borden-kill?page=show