Because instead of just remembering that Spanish people siesta from 1200-1400 local time, you might as well translate it to UTC and make it even more complicated to remember?
It is possible that it appears complicated because it is not common to use UTC. But let's try to imagine the scenario further...
So Spanish people typically siesta between 1100 to 1300 UTC. Imagine, my home country is UK. I work between 0900 to 1800 UTC. Imagine I travel to US and need to call my Spanish friend. In the US, when I reach the east-coast I realize that I work between 0200 to 1100 UTC. At once I can deduce when to call my Spanish friend; it is before I leave my work at 1100 UTC.
Frankly, even I had not thought through completely the UTC based scenario, until now. I am now even more convinced that it's the most convenient way forward.
When does your Argentine friend siesta? When does your Japanese friend get off work? What's a good time to phone your friend in Australia? You've just landed in Tokyo--when should you be getting to sleep? You still have to do time zone calculations, but you don't have the time zones anymore.
