Please explain, if the ketubah is a binding document, with witnesses signing that the husband made certain promises to his wife, then if he fails to fulfill those promises why doesn't that automatically result in a get al pi halacha?
Indeed the ketubah is a binding document according to Jewish Law-the halakha. However, there are two points that you may think exist, which do not do so in practical terms.
1. There are obligations of behavior that come together with the ketubah but they are not enforceable. There actually is no practical penalty clause if these obligations are abrogated. Basically, the obligations in the ketubah are monetary--regarding wife support throught the duration of the marriage and in the event that the husband decides to divorce his wife without just grounds for divorce. In Israel where the Rabbinic Courts are part of the legal system, he can be obligated to pay wife support. However, the Rabbinic Courts in the diaspora have no such power. Nevertheless, even in Israel, the promise to pay a lump sum in the case of a divorce is almost never put into effect. Halakha has evolved today so that any wife who agrees to or asks for a divorce does not receive the ketubah sum. As I mentioned, the sum is to be paid if the man unilaterally decides to divorce his wife without proper grounds for divorce, against her will.
2. There is no mechanism in the ketubah contract that addresses the situation where the woman demands a get. Even if she demands a get due to the faults proven in the husband, the ketubah does not include provisions to help her get her divorce. This issue is addressed in the talmudic and rabbinic literature and rulings--but the bottom line is that today a Rabbinical Court is unable to "issue" a divorce decree unless the husband willingly gives the wife a get.
This explains how the halakhic prenuptial agreement (The Beth Din of America's prenup at www.theprenup.org or the "Agreeement for Mutual Respect" in Israel at /www.iyim.org.il/prenup/) is a natural extension of the ketubah. Just like the ketubah, it comes to fill a need for protection of women at times of the dissolution of the marriage - in the circumstances that exist today in Jewish society. The prenuptial contract for the prevention of get-refusal does not come instead of the ketubah contract, which still stands in every Jewish marriage -- it expands upon it.