I'm inclined to think that in order for another search engine to thrive, there needs to be a segregated market created by a different language. The kind that creates a culture gap that can't be bridged easily.
Ex - Google dominates the English-language web primarily. Most Europeans, esp Western Europeans, while they may have their own languages, speak enough English that they can get good results from Google, enough better than any local competitor that they can't get a foothold easily. Thus, they tend to use Google, enough to help Google get pretty good results in their own language too. Russian, though, has a ton of native speakers who speak no English. It makes more sense for them to gravitate towards a search engine where Russian is a first-class citizen, for searching the Russian web. All of the energy from the Russian-sphere thus tends to go to that engine, and Google's presence gets neglected and gets worse. Ditto Chinese, etc.
Ex - Google dominates the English-language web primarily. Most Europeans, esp Western Europeans, while they may have their own languages, speak enough English that they can get good results from Google, enough better than any local competitor that they can't get a foothold easily. Thus, they tend to use Google, enough to help Google get pretty good results in their own language too. Russian, though, has a ton of native speakers who speak no English. It makes more sense for them to gravitate towards a search engine where Russian is a first-class citizen, for searching the Russian web. All of the energy from the Russian-sphere thus tends to go to that engine, and Google's presence gets neglected and gets worse. Ditto Chinese, etc.