The voltage levels, particularly at the high-voltage and extra-high-voltage levels, have long been standardized in different industrialized countries. The North-American system is 60 Hz, while the European system is 50 Hz. Each other country in the world (as far as I know) follows one or the other frequency standard. Japan is the only country I know of that has both 50 Hz and 60 Hz systems.
Then, once you've picked your "system frequency", your life would be a lot easier if you follow either the European nominal voltages (for 50 Hz systems) or the North-American nominal voltages (for 60Hz systems).
Simply because these are the largest systems and therefore the major markets for the manufacturers. So the manufacturers already have these nominal voltage levels embedded in their designs.
Obviously, these large networks would not change their nominal voltages to achieve a "world standardization". You will never see the 500 kV network in North America modified to operate at 400 kV, to follow an "international" standard. And if you suggest that the rest of the world (380 kV and 400 kV networks) be upgraded to 500 kV, it will be called "imperialism".
So, call it the "practical standardization" or the economical power dictatorship, but the reality is that these voltage levels are already standardized, worldwide.
Of course we need specific voltage levels in generation, transmission and distribution for equipment design. Of course we cannot follow a new voltage level every time for electrical equipment design for different country or area. Better to follow the already adopted voltage & frequency levels by Britain, USA & Japan etc.
Of course voltage levels for generation, transmission and distribution are decided on the basis of certain criteria where economy is one of the most important factors. As there is no doubt that the best AC power supply can be obtained from sinusoidal voltage waveform so the designers have to use it for a variety of situations. In many countries of the world the transmission voltage levels are not more than 500KV but in Russia, USA & Canada it is 750KV or even up to 1000KV as the resources, distances and climatic conditions in these countries vary from other countries. In many South Asian countries distribution voltage level is only up to 220V whereas in many Arab countries, USA, Europe, Japan etc. it is up to 110V as well. Therefore we may conclude that engineering formulas, calculations, economy and demographic situations decide voltage levels.
However once decided certain voltage levels must be followed to avoid any miscreation in design of electrical equipment. For example Mr.ABC purchased a washing machine which was designed by some company to run at a voltage of 400V but only 110V supply is available is his house so he has to throw this machine away or return it back to the shop and get a new w/machine designed at 110V.