Improved balance.

Out of personal interest, I'd like to ask: why won't it be displayed? I'm just curious.

Because the upkeep cost can only be implemented through Lua scripts. The income forecast in the kingdom window only takes into account what the game engine itself tells it, without any scripts.

No, I heard that the number of resources was reduced in Heroes 6, but I didn't take the idea from there. But still, I want to say that the resources are somehow illogical; for example, mercury is needed to build buildings. Maybe this is one of those features that have been in Heroes since the beginning.

Different resources are needed so that the player collects them specifically (for certain buildings), and not all at once. For example, infernals most often need mercury and sulfur, so they will look for mercury and sulfur on the map, and not eat everything in a row.

I admit, I didn't understand what the troop upkeep and game dynamics have to do with it.

Troop upkeep slows down the game. It requires more time per turn (the player has to calculate their expenses themselves, predicting subsequent moves). It requires more turns to gather and maintain a stronger army - unless, of course, the cost of hiring units is reduced in parallel with the upkeep.