Recently I heard a different reason suggested as to why there are fewer and fewer tenure-track jobs in philosophy. University administrators are taking control of the tenure review process; previously departments made decisions and the administrators rubber-stamped them. The result of this is that it is easier to get tenure. This is because university administrators grant tenure based on quantitively-measurable achievements, rather than a qualitative assessment of the candidate qua philosopher. If a department thought that someone shouldn’t get tenure, the administration might turn around and say that they are going to grant it because the candidate has fulfilled such-and-such requirements.
Since it is easier to get tenure, hiring someone at the assistant professor level is much riskier for a philosophy department: they have to assume the candidate will get tenure. So the pre-tenure phase is no longer a probationary period. That is being pushed onto post-docs and graduate students. This results in the intellectual maturity of published work going down.
There are various assumptions in the above that could be questioned, but what’s interesting is that it takes a lot of the blame for the current situation off the shoulders of faculty members (there have been accusations that they are not doing enough). If tenure-track hires are a bigger risk for the quality of the academic philosophers who end up with permanent jobs, it is good that they are averse to that risk.