The class event is certainly most interesting for the low levels and beginners!
Who comes up with the idea of making such restrictive restrictions that there are only 11 units to choose from in today’s class event, even for advanced and professional players?
So how many units do beginners have to choose from? Three, four, five…?
Who makes such decisions at the manufacturers… a BOT, an AI, or an apprentice because everyone else is on vacation?
It’s the Monk class trial.
Monk is troop type Elf and kingdom Shentang. That’s where the restrictions come from.
So your question should probably be, “why are there so few elves from Shentang?” if they are supposed to be a thing? 
Most of the event paradigms have long-standing restrictions:
Raidboss is limited by kingdom. There was a change relatively recently in which the Hero was similarly restricted solely to weapons from that same kingdom; it used to be that the Hero could use any weapon. Heroes are not restricted to the class of the given kingdom and may instead use any class the player has access to.
Underspire is also limited by kingdom, and Heroes are restricted to weapons from that kingdom, but may use any class they have unlocked without the kingdom restriction.
Invasion is limited by class. Heroes may use any class they have unlocked without the same restriction that applies to other troops.
Tower of Doom is restricted by a single color, including Hero weapons.
Delves are restricted by two colors; a troop must one one of the two selections to be permitted within. Heroes are not held to the same restriction and may use any weapon and class without limits.
Class events are dual restricted; troops must match both the kingdom and the class in question. Some kingdoms are considerably better than others insofar as quantity and quality of available troops; Shentang is among the worst offenders in this category. Heroes are completely unrestricted, both in terms of Class and in terms of weapon selection.
World Events are a bit of a hodge-podge; each event has different restrictions and regulations. Troop selection is constrained by whatever limit(s) are placed on the event; Heroes are usually unrestricted but will be constrained to a small(er) set of weapons if there is a kingdom or color restriction.
PvP varies by region, similar to Raidboss, Invasion, or Tower of Doom outside of the Central Spire.
It’s pretty much done by a set formula, now, and outside of World Events there isn’t variation or room for creativity.
Troop Type, technically, but yes.
The Hero being exempted from most restrictions has ALWAYS been slightly nonsensical…
