The problem with coordinates is that they don't necessarily cover changing political centres. The opinion of the true legal time will differ based on political calculations. In a territory that is internationally recognised as Ukraine, annexed by Russia but under the contested control of Ukraine, how does the coordinates help? They probably don't. You'll have to know whether that particular time was from the Ukrainian perspective or the Russian perspective. In most cases that will be easier to code by a tzid than geographical coordinates.
Another example is Xinjiang province, where in some towns everything is in local time, in some towns everything is in legal time, in some towns it depends on if you're Han or indigenous, and in some towns it depends on what you're doing (bus timetable vs shop opening hours).
Another example is in a very large but lightly populated part of Australia, where there local community uses a defacto timezone (Australia/Eucla) but the legal timezone is different (Australia/Perth) - it is no use saying that the polls close at 6pm and the general store closes at 6pm and coding them with the same coordinates, because one will refer to a moment 45mins after the other. Other places may have similar variations of opinions where the formal boundary of two timezones isn't really obvious - I have heard that the timezone boundary around Broken Hill is a matter of opinion as much as it is a matter of law.
The other possibility is of course that perhaps in a country like the US where timezone boundaries are relatively unstable over time because they pass through states. It's true that coordinates could be used to deal with it. But it's not obvious that this is such a systematic and regular form of variation that timestamps should use coordinates, especially considering almost any record that relates to time in a place probably already codes the place so an ad hoc migration should generally be possible. (In such a case, I would still like tagging the time with a tzid so you know for a fact whether it is premigration or a postmigration.)
