1. It's a natural Schelling point.
If I name a random Asian country, would a typical American know what time zone that country uses? If you name a random US state, would I know what time zone that state uses? The answer to both of these questions is "no".
What is reasonable to expect people to know is their own time zone, specified as a UTC offset. I know exactly how many hours local time differs from UTC, and you should too! If you ever have to communicate with someone on a different continent (for personal reasons, business reasons, or otherwise), knowing your own time zone is practically a prerequisite.
When is midnight UTC in my time zone? I just add the offset and know immediately. I don't need to convert twice, or take the difference between two offsets.
2. No DST troubles.
Daylight saving time is not uniformly observed, and even in the locales that use DST, DST activation/deactivation don't happen on consistent dates worldwide. Again, as above, if you know your own time zone offset, then you know how to convert midnight UTC to local time. But for a time specified in a DST-observing time zone, you would have to know whether DST is in effect or not at said time zone. If your own locale also observes DST but at different dates, now you have four offset changes to keep track of.
Granted, any non-DST-observing time zone would work as well as UTC for this purpose, but… why not just UTC? (See point 1.)