That is good example why one timezone doesn't work. The locals in Xinjiang use a local time zone +6, instead of China time +8, because the latter is too far off the daylight hours.

My understanding is that use of Xinjiang time has dropped recently because of the crack down on Uygurs and government forcing China time.

There are various countries that optimize the number of time zones for administrative purposes, but this is much easier and sensical to implement within one country than globally.

UTC is used globally when it makes sense, e.g. for the schedules of international radio broadcasts.