What makes securing faculty positions difficult? This question is an off-shoot from this one, where it has been agreed by most that securing faculty positions is difficult in general. I would like to know what exactly makes this so.

Though in theory university rankings may be pointless, there is a broad quality-based classification of institutions in any country which many will agree on - for example, the crème de la crème, top tier, middle tier and decent universities, of which there could be a few hundreds. We shall assume the student has passed out with a good thesis and impactful publications.


Is it tough for a student graduating from a higher rung to gain a position in the lower rungs? 
What factors dictate the difficulty in securing a position in a university in the same league?


PS: In India, the answer to Q 1 is "not at all", as there is a heavy crunch for faculty positions even in top institutes. Instead the difficulty arises only when students from low rung colleges seek top positions: in most cases, such students are found wanting in skills.
One of the strange effects of faculty hiring (and graduate admissions) is that offers do not necessarily go to the strongest candidates.  Departments have limited resources to interview, recruit, and hire faculty.  Interviews are expensive; startup packages are really expensive; faculty job offers burn political capital even when they aren't accepted.

So hiring committees make strategic decisions based on the perceived probability that candidates will accept the position.  The University of Southeast North Dakota at Hoople would most likely not interview superstar applicants, because they don't want to waste their time interviewing someone who's "obviously" going to get offers from stronger schools.  As with any self-selection process, this assumption is partly justified and partly Institutional Impostor Syndrome.

So no, selecting an MIT grad is absolutely not a no-brainer for U-Cal-XYZ.

And yes, sometimes reasonable PhD students from very strong schools fail to get faculty jobs, or even interviews, because they don't quite have the research record to get an interview at the best departments, but their pedigree scares off weaker departments.