The whole point of having several resources is to force you to balance their use, that's part of the strategy. It's not like your opponent doesn't have to worry about these things, they also have to balance the use of rare resources.
No, the point is, factions in this game tend to use different distribution and different total amounts of resources to build up their dwellings. The strength of each factions factors in hero abilities, spell access, unit qualities, and expense of reaching each unit. And half of that stuff is basically randomized, meaning that every town needs to be viable on a variety of map designs without players getting particularly screwed over with bad arty/resource/level up placement or the converse and getting a gamebreaker combination.
Throwing in unique resource usage adds another variable that interacts with every other one of those variables to this already very complex system, that if I need to remind you, has never actually been balanced in any of the previous installments. You need to make sure that the new resource usages are balanced between factions if they are made different, and balanced against common usages between factions such as buying spells, etc. You need to make sure that early vs. late game balance isn't significantly impacted by these new tools. You need to make sure that both heavy utilization of these resources and traditional Heroes economy/army build up focus don't obsolete each other. Etc. and so on.
All of this demands playtest time, which this company might not have given Ubisoft's history.
There is also the ever eternal question of trying to draw in new players while retaining the old. No company survives on an existing fanbase with every sequel because there will be invariably players who hate new changes, and no company grows without attracting new fans. While ideally a game should retain enough complexity to keep the old fans occupied and be accessible enough to attract new people, when you are ultimately given the choice between burning your company down "for the art" and a chance to survive as a company so you can do the "art" thing later, the choice is obvious to sane people.
Don't forget that this is Ubisoft we are talking about, one of the less nice publishers out there. And be mindful that whatever changes to make the game more mainstream have been made, it could have been a lot worse. Ubisoft was seriously considering making HoMM 5 an RTS or an RPG for sake of broadening the appeal.
