...even though they're supposed to properly evaluate you and make an objective analysis of whether you are a good teacher or not, too often it becomes a political thing..and you become a political pinball...maybe the principal is black and you are white and the decision is based on skin color (or vice-versa, or latino v white, et al, or principal thinks you're too conservative, or too religious, etc.
And of course they will NEVER say that's why they aren't going to give you tenure, they won't say anything...because they can't "non re-elect" you for "any reason or no reason..." . As a teacher coming out of college you assume they will be reasonably honorable , but then you find out it can be a very SLEAZY and dishonorable system.