Personally it is because I know I would eventually disrobe, I would not be able to keep the precepts. At one point I had thought, very strongly, that I wanted to ordain. But taking a good, honest look at myself I can see that I am not ready. I would only want to ordain if I knew I was going to be there for life.
"If beings knew, as I know, the results of giving & sharing, they would not eat without having given, nor would the stain of miserliness overcome their minds. Even if it were their last bite, their last mouthful, they would not eat without having shared."