They have made it too difficult to get. Why should you move across the country to a less than desirable job on the theory that it will be stable and will be a space in which you can do your work, just to be tossed out — particularly when it is not a job that paid enough for you to save, and that you may even have gone into debt in order to keep?
That is why there are so many people who do not want tenure, but only long-term contracts and good salaries. Then they do not have to move to the wilds, to the one tenure-track job they can get, and they are also independent of the tenure system. This is considered rational.
I am against it because by abandoning a tenured post you abandon the university to the administration and the staff, and you convert faculty into employees and contractors. That, in turn, means the university is not a university.