Yandex is mainstream, it's not a geeky site, and most Russians don't speak English well enough to prefer originals to translations. So most of its users just don't arrive at the limits of Yandex's index ever.
Of course it's nice for me to be able to go to google.com and search in Russian or in English or in any other language (and get results in the language I search), but it's just not what the majority of Russians needs or wants.
Their policy on the search index was 'only russian-language content' (their bot analyzes page text rather than looks at the TLDs or IPs), then a couple of years ago they started including more and more sites in english, starting from encyclopedias etc. This is possibly also because their indexing technology was deep into morphology of Russian language from the very start in late 1990-s.
PS their slogan is a bit more dramatic - 'Everything WILL be found' :)