It says turn, but it probably means your turn.
If you want to apply this spell from a strict and literal reading of the rules, then sure, every creature makes the saving throw every turn.
However, this creates a highly unusual phenomenon: the power of the spell scales with the number of combat participants. Every creature that rolls initiative in a combat would increase the number of saves per round made by a victim of the spell. This makes exactly zero narrative sense. Given this, it seems clear enough that the spell should say something like “at the end of each of your turns”, so that the number of saves a creature makes per round is independent of the number of participants in combat.
So the way I would rewrite the spell description, and the way I have always run this spell, is:
When you cast this spell and at the end of each of your turns that you spend concentrating on it, each creature on the ground in the area must make a Dexterity saving throw. On a failed save, the creature is knocked prone.
Now, there is a rule we can draw support from to motivate this ruling - a combat round is always six seconds:
The game organizes the chaos of combat into a cycle of rounds and turns. A round represents about 6 seconds in the game world.
The idea here is that a round is always six seconds no matter the number of participants. Because of this, we can see that narratively, each individual turn in a round actually is happening at essentially the same time. In the fiction of the game, each creature’s turn is not a discrete event at a different time, all the turns are occurring at the same time, with initiative representing how quick each participant is to act in each combat round.
Thus, we should expect each casting of earthquake to produce a similar effect requiring the same number of saving throws made every six seconds that the spell persists. The idea that earthquake produces an arbitrarily large magnitude of effect scaling with the number of creatures that happen to be participating in combat is simply inconsistent with the narrative the combat rules are trying to represent.