I'm not sure why Maps is dark-mode only on the watch. It is the general theme for the Apple Watch and all of its apps (not to mention the Watch app on the iPhone).
It doesn't generally bother me - I'm almost using maps on my phone anyway, and just use the watch to alert me to turns, etc.
It may be because white pixels on OLED use marginally more power than gray pixels, though that's probably not really much of a difference in reality. But Apple is always trying to reduce power consumption on the watch as much as they can, so maybe that's part of the thinking. Also, I think watchOS gets minimal attention these days - I think it's just being maintained and now getting ready for Siri AI.