Midnight or Noon are easy to track when they happen, and are measurable any time of the year. Sunrise varies, so you'd have to accurately track when the equinoxes are, and then accurately measure when sunrise happens, and then have an accurate clock to keep that time for the other 363 days of the year. We've only reliably been able to do that for a couple of hundred years. The definition of 'day' predates that by centuries.
(By the way: In tropical areas, it's not uncommon for the historic 'day' to start at sunrise. Of course, sunrise varies by at most an hour over the course of a year for many of them.)