As a simple example:
* Start by optimising "players can always do something on their turn" -- but that just ends up with everyone always having exactly one thing they can do (no choice).
* So then say "give players more things to do each turn" -- but then they end up being able to do everything every turn (the game gives them too much 'money' (still not really a choice)
* OK, so we want to force players to make a choice -- so we say "No, give players as much choice as possible, but make sure if they choose an option it blocks off others (in practice, make as many sets of maximal tasks as possible)" -- but then the AI will make sure every turn every player can do (for example) exactly 3 out of 6 things (any 3), and make sure no matter how well or badly they play they still always get to choose 3 from 6, so the game doesn't really progress, or vary.
So, what we want is choice, but also variability, and progress, and players to feel like they are effecting the game, but also don't let one player run away too early, but also don't make it just "feel random who wins", etc.