First... I have Tropico 5... IMHO it was a step back from Tropico 4... IE: It lacked the election speeches, which I think it nice way to start this discussion with one thing I truly appreciate Tropico 6 brought back into the franchise, the Election Speeches (what is a President without them).

On to the purpose of this post:

Tropico 4 (and nearly ALL others) had a "scenario setup" even in Sandbox that tweaked the gameplay in meaningful ways that helped keep the game from growing stale.

(*) You had good traits/bad trait: that tweaked the playstyle (in T6... we have just one).
(*) Random Event Frequency (I don't mean disasters) gave me a good random twist of challenge that kept my game from getting stale - surprise challenge and tweaks I would have counter, being resourceful - kept those sandbox games fun and a sudden burst of direction.
(*) Even more consistently meaningful, every sandbox game had a trait of its own
IE: Faraway - less tourists, but less intrusion by superpowers; Installed by CIA; Elected as Capitalist; Heir apparent, ETC ETC ETC ( alot of fun choices, in T6 NO choices).

These traits, especially "how" you become Presidente added both persistent challenges and persistent rewards that gave the player more direction in their game, while still giving them the freedom to play it as they would.

When T6 gets monotonous (the SAME EXACT Tropico sandbox every game), I return and play Tropico 4.