Tenure is just one of many forms of dependable income that is harder to get now that it was a few decades ago. I suppose it's because there is more competition: more people at university in the first place, more students per professor, and people who are profs started as students. Universities have also figured out that they've got you by the balls once you decide you want to try the tenure track, so you end up doing a lot of postdoc before you get that prize.
I sound like Karl Marx himself when I say it, but it's the capitalists extracting as much work as possible from the helpless workers. (I don't think I'm normally even on the left, but here we are.)