The troop limitation for tournaments is alway one below the hero limit; i.e., 3* heroes and 2* troops, 4* heroes and 3* troops, and 5* heroes with 4* troops.
Why? One answer may be that there are no five-star troops.
Another answer is that it enables newer players to compete without the disadvantage of not having leveled up four-star troops. It takes a while to even get a 4* troop, much less level it up.
A cynical answer is, by requiring varying troop levels for tournaments, SG increases the likelihood that we will invest more in “lower” troops for it’s ripple effect on revenue – troops require a lot of food, and that likely translates to gem purchases for that food.
Perhaps I misunderstood the question in the OP. Is it “Why do tournaments restrict the level of troops?” or “Why aren’t there mana troops lower than four stars”?
Originally there were no 4 star troops when the game was first launched, hence there was no concept of mana troops. 4 star troops were added later on, while all other troops (3 stars and below) remained the same. Atleast that’s what I know.
They rebalanced mana heavily in v1.4, especially fast and slow heroes, defense team mana from combos, 4* troops and introduced the first mana control hero ( which was also the start of the 5* HotM business model ).
Notes
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The whole thing is a very interesting read, but lacks any context:
What about creating of new 3 star mana troop? We see only new 4* troop, but the game is not only 5* heroes and 4* troop. I thing that will be really interesting if EP considers the option to create new 3 * troop that increases the speed of mana recovery.