I kinda start to realise it why. If I'm right, its because if it was instrumental, it would be associated. So like "today is Monday" and today is always monday, but thats not true, and thats why its "Dzis jest poniedzialek", right? So basically instrumental only used with nouns?
I think it is beacuse "dziś" is not a noun but an adverb.
So we have adverb is noun (or noun is adverb but in changed order)
Dzisiejszy dzień jest poniedziałkiem has the same meaning. (but adjective+noun )