Why do some countries choose not to make a language official?

Perhaps specifically *because* it is already the de-facto language of the vast majority of the population. There is no challenge to the language's official status, and therefore no reason to make it legally official.

You might find that governments that choose official languages do so in response to political or social pressure to choose an official language for administration and education. If you lived in a country where more than one language was widely spoken (historically), you might be forced to choose which language would be used for things like official government announcements, textbooks, testing, etc. (think Canada).

Or, you might live in a country where there were many, many dialects, but one group (speaking one dialect) came to power and chose its own dialect (and perhaps others) as the official "language(s)" of the administration and the nation (think China, Japan, and India)

The traditional English answer to any problem is to do nothing in the hope it will go away. When William of Normandy conquered England, French became the de facto language of the country, but there was no point in courting trouble by making this "official." The chaos caused by the Black Death, followed by the War of the Roses allowed English to reemerge, then our first printer, William Caxton saw a commercial opportunity in promoting English (which would allow him to develop a new market safe from the import of cheaper foreign made books in French and Latin, and finally the Protestant Reformers promoted a liturgy and an official Bible translation in this new national language, the linguistic change over was practically complete, but there was no need to make this legally explicit, so the royal mottoes and the royal approval to new acts of Parliament could remain in Norman French... As England had never made English official, there seemed no need to the rebel colonists in North America to act differently. In fact Congress actually debated whether to adopt some non-British language and some Native American language or Athenian Greek, the language of the first democracy were both suggested, but Congress failed to agree so the debate was adjourned without a decision, and it was simpler to leave the question in limbo than to waste time trying to come to some positive decision.

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Canada has 2 official languages. Some other countries have 2 as well. Why English is not official U.S. language?...Because when 'founding fathers' were debating that subject, Ben Franklin wanted Latin as official language ( and turkey as national bird.), others wanted German, still others wanted English...They just could not agree on what language to make official, and subject remained unsolved til today.

Even though the majority may speak one language, I guess it's for respect out of those who speak the other languages.