Why is research at highly productive research institutions better than those at not-so-highly productive research institutions? As of time of writing, the University of Oxford is top of the Times Higher Education world university ranking list, while the University of Southampton is 118th. (Nothing special about the THE, Oxford, or the University of Southampton - I'm just picking examples.)

Because a university's research is usually a major factor in these rankings, an immediate implication of this is that as a whole, Oxford's research is "better" than Southampton's. Why is it better? I can think of some explanations:


Professors at Oxford are smarter than those at Southampton. Obviously the smarter you are, the better your research.
Professors at Oxford are more motivated than those at Southampton. Maybe they work 24/7 while those at Southampton work 9-to-5.
Professors at Oxford have more resources than those at Southampton. For example perhaps they have more + bigger grants they can apply for, and with more money, they can hire more students + conduct experiments using equipment their counterparts at Southampton don't have.
Professors at Oxford are luckier than those at Southampton. They "just happened" to pick topics that later led to massive advances and Nobel prizes, which made them academic superstars. Oxford then gave them very attractive job offers to lure them there. Now it's a rich-get-richer situation - once you are famous, other people read + cite you more too.


Are any of these explanations accurate? Are there other factors I've not thought of?
Your question is a bit circular. The "best" universities are at the top of the list because it's a list of the "best" universities -- however that is defined. :)

What makes a "top" university is largely a rich-get-richer feedback loop: The "best" universities attract the "best" researchers, which makes them the "best" universities. The "best" researchers get the most funding, which helps them do the "best" research. The "best" universities get the most money, which helps them spend more on research.

That explains why older universities tend to have higher rankings. They have had a long time to gradually build up that feedback loop. However, external factors have a big impact -- availability of funding and university management is important, as is the desirability of the location/country. Countries like the UK and US have a lot of good universities because they are attractive places to live and their government provides adequate research funding and, importantly, this has consistently been the case for decades or centuries.

One important point you are missing from your list: Some professors are teaching six courses per year -- they have no time for research. Other professors are only teaching one course per year -- they have plenty of time for research.