C J Campbell writes:

You see that all over Asia. People who supposedly speak the same
dialect who cannot understand each other, so they speak English.
English has become what Esperanto was intended to be.

Chinese is a special case because the spoken versions of Chinese are mutually
incomprehensible; they are completely different. At the same time, the
written versions are generally coherent. So one Chinese person may have no
idea what another Chinese person is saying, but he will immediately understand
if they communicate in writing on a piece of paper.

This is a consequence of the Chinese use of symbols for concepts in the
written language, rather than symbols for sounds. The written language
provides almost no clue to pronunciation, and so pronunciation drifts until
the spoken languages become completely separated.

It's also one reason why Chinese is not likely to ever become a universal
language.