Each language needed its own code page. For example Greek uses 737 and Cyrillic uses 855.
Cyrillic is not a language, it's an alphabet/script. Codepage 855 was used for Cyrillic mostly in IBM documentation. In Russia codepage 866 was adopted on DOS machines, because in codepage 855 characters were not ordered alphabetically.
Even today, on modern windows machines, typing alt+163 will default to 437 and print ú.
It's only true for machines where so called "OEM codepage" is configured as codepage 437. But in Russia it's codepage 866 by default, so typing alt+163 prints г.