Because it's the middle of the day, not the day of the middle -- it's a kind of middle, not a kind of day.
"middle day"? That sounds like a description of Wednesday (the middle day of the week in some calendars).
tagmezo is a kind of middle; it's not a kind of day.
Ohhhhhh I see, you'd have to say 'meza de tago' to make the concept I was thinking of huh? Or actually 'la mezo de tago'. We'll thanks lol. Clarifies some things in my head.
I am Russian and i don't understand the difference between cases of using and not using of word "the" with day parts in sentences "Morning is before noon", "The morning is better than night" Mi pets helpon!
"The/La" is used to indicate a more specific morning. "Mateno/a morning" could be any morning. In my experience, this usage is used for talking of the concept of mornings in general. "La mateno/the morning" is one specific morning, as is usually revealed by context. I hope that helps.