Language is and always has been a huuuuge point of nationalist pride. Linguistic purism, the notion that one's language is more pure or authentic that those around you, has often been a capstone of that sort of thing. You can see this anywhere you go where there's something perceived as unique. It's part of the narrative of the greatness of Icelandic, that the sagas are still readable today to the average person (skipping over the fact that this kind of ability is learned), and how Icelandic is more pure (which is in part a direct response to the rejection of Danish influence on the language, and a push toward to what were at the time seen as more conservative forms of the language spoken by lower-prestige speakers.

Chinese is the same way. You can not spend more than 5 minutes on a Chinese language forum online without seeing a post about how Cantonese (or Hakka, or Wu, or whatever) is more authentic than Mandarin, and "all the old poetry sounds best in [whichever language]" because of this notion of purism. The reality of course is that even languages like Cantonese, which might be conservative in some areas, are quite innovative in other areas. No modern language is as-a-whole the same as its ancestral form. Anyway, point being, the notions of linguistic purism and the ideas of prestige tied to different language varieties is a global and pervasive feature of human societies at least as long as language has been recorded or talked about.

Languages which spread are also tied to culture in really significant ways. Not just that language is culture, but rather Culture™ as the idea of progress, development, intellectualism and the like. The "we're more cultured than them" sort of culture. So if your language spreads, especially as one with a written tradition surrounded by languages with weaker or no written traditions, that's just further evidence that you're better. A lack of a written tradition is also tied to lower prestige, even among people in those situations. Even if spread doesn't happen (Korean*) it's still a huge point of national pride that ones language is standardised, or a writing system is invented (even if it's not widely adopted until centuries later, again Korea). And if that was created/sponsored/encouraged by a monarch, even better.

There's a silly debate had by non-linguists (I don't know any professional linguists in the academic tradition who take part or particularly care) about what a language is vs what a dialect is. In the area where I work, with a bunch of tiny tiny languages (~1000 speakers or less in many cases) and no written forms, the distinction is simply one of writing. It's a language if it has a written tradition, and a dialect if it doesn't. This isn't the usual way people make the argument in English speaking countries, but it's pretty telling about the power of language as a tool of spread and conquest. It's also caused a lot of people in the region to start developing their own writing systems, with differing degrees of success.

tl;dr: Language, or well the ideas around language, are wicked powerful both for members of a community and those who seek to rule over them. For good and for bad. The spread of ones language is quite simply often seen as a sign of a better-ness of that language over others. And these are ideas which remain today around the globe, that some languages are objectively better than others (they are not) or objectively able to communicate more (nah, unless we're talking specialised vocabulary which is often borrowed anyway and can easily just be borrowed again), or are objectively more information-dense or ambiguity-preventive (again, nah).

But the perception is there, and that's what matters.

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^*Cia-cia ^propaganda ^and ^possible ^later ^actual ^adoption ^aside