Counterpoint to this website: it is convenient to have symbols to represent things. I don't see them suggesting anything better than flags.
Well, why not use currency symbols to represent language? Same amount of correctness. If you like flags because they are colorful and memorable icons, you can use them to represent time zones. Much more convenient than some obscure "UTC+7" or "PT", isn’t it?
Because a currency symbol is less correct (think $) and isn't as recognisable. Flags have colours that make them easier to pick out, and can be displayed at much smaller resolutions.