[reaperducer]

That's the problem that web directories solve

Not really. A web directory is a directory of web sites. I can't search a web directory for content within the web sites, which is what a niche search engine would do.

[Beldin]

On the other hand, the niche search engine depends upon having such a web directory (the list of sites to index).

[teddyh]

Like WAIS?

https://en.wikipedia.org/wiki/Wide_area_information_server