The top professors tend not be judged on their research, in my experience. They're judged on their publications. I, personally, know many 'top' professors that have enormous publication lists that come from being the guy who supplied the money via a grant application. From their publication list - which is what 'proves' their research excellence, you would be forgiven for thinking that they were savants. But they're generally just administrators with an eye for the main chance who view the publication list as yet another thing to be gamed.
Publications (or more generally, H-Index) are used as a proxy for research by administrators that cannot possibly judge the merits of the research itself. During the tenure process, administrators use other signals too: recommendation letters from peers, grants, "service", graduated students and their placements, etc.
When your work is only relevant or approachable to less than a dozen people on the planet, how can you expect a dean to judge it? They can't. So they use (often poor) proxies.