I wonder why the Wizarding Legal System doesn't use Unbreakable Vows in their legal system. Having the judges swear "to uphold the law to their best ability while acting in their official role" would really cut down corruption, making it almost impossible to abuse ones position.
Likewise, having accused criminals forced to swear "to not intentionally commit perjury while before the Wizengamot today, and to vocally correct any mistakes in [accused's] claims when realizing a mistake has been made". This would make it a more limiting Vow, as it only lasts for the duration of that particular day, and could be used for any trial about a crime of specific severity.
Most likely, it's not used simply because the Wizarding World is based on, and created from, corruption and power-lust. The very goverment exists more for the sake of being able to obtain power than to create a good country. The members of the Wizengamot want to be able to do illegal and immoral acts to further their own power, so they refuse to make laws that might limit their ability to do so (or increase the risk of them geting caught).