You and the lady's family have my condolences.
To answer your question: Because of the amazingly depressing nosedive it can put one into.
As mentioned in a previous thread, witnesses are constantly told that they are not good enough, that they are sinners, and people who are not good enough will not go through Armageddon. Also, (again mentioned in another thread) witnesses are often made to feel helpless, like they cannot change anything, so in their minds, they will never be good enough.
For instance, when I was 16 I decided I wanted to try to get baptized. I had some social anxiety problems, so it was very hard to go out on the ministry. I went out a few times a month anyway, and then requested baptism. A week later, the elder came back and told me I had to get my hours up. This made me quite depressed, but even more determined to push on and prove myself good enough for Jehovah. So, I tried harder through these depressive times, and managed to almost double my hours. I requested baptism again. Again the elder came back to me and told me my hours weren't high enough. He then pulled out a sheet of paper, and my report record, and proceeded to show me I was still under the average for publishers. So, in my mind, not only was it extremely hard for me to go out on the ministry, even when I gave 100% it still was not enough.
I remember that night praying on my hands and knees that Jehovah would give me something to live for and prove he cared about me, or kill me, and even said I would kill myself. I was crying rather hard, and was very close to doing something stupid. It was at that point that my life started to turn around... just a short time later I met the woman who is now my wife (the elders told me not to date her at all, but I ignored them), and a friend who I had not heard from for quite some time called me and asked if I wanted to stay with him and his family in Devon. I firmly believe to this day that both of these events were arranged by Jehovah, because he knew that if the organization had kept me any longer, then he would have one less follower.
The organization's writers and speakers are well versed in psychology. They did not go to classes and college for it, but they learned from those in the congregation that brought them into the organization. They are constantly put down, and made to feel inferior, and that no-one else wants them, even Jehovah!
Quite simply put 99% of JW suicides are essentially caused by the murderous bastards that tell us we are worth nothing in God's eyes..
FMZ