I know that Google has a few alternatives such as Bing and DuckDuckGo, among others.
However, back when I first got into SEO, there were a dozen search engines: GoTo.com, Yahoo!, Excite, Lycos, etc. They each brought a decent amount of traffic.
Why is it that Google has become such a monopoly that not a single other search engine can gain any reasonable share of the market in all these years?