Basically, because most languages already have their own words for numbers, and no particular incentive to change them.

A lot of languages have used base 10 for a very long time; it is a pretty natural number system to settle on because we have ten fingers. So, for example, Proto-Indo-European, the reconstructed common ancestor of Indo-European languages dating back several thousand years, appears to have used base 10.

Conversely, there are still languages today that use other bases. Here's a discussion of various linguistic numeral systems, along with a map showing their distribution. Mostly base 10, as you can see.

There was never any point where base 10 was officially established worldwide per se. Are you perhaps thinking of the metric system? In that case, units that scale with base 10 were indeed standardized, though of course things like pronunciation and plurals vary between languages. So an English kilometer is a French kilomètre is a Spanish kilómetro is a Bengali Kilōmiṭāra is a Swahili kilometa, etc etc. But in this case we are talking about adding new words for something that did not exist in the languages before (e.g. a specific distance), rather than replacing pre-existing and commonly used words. That first type of change is much easier than the second.