Only because we are currently in a period directly following a patch that nerfed Zerg and buffed Protoss. Once the ladder and meta settle, Zerg’s “overall” win-rates (which are not an accurate representation of balance in the first place) will be inline with the other races (fluctuating slightly between higher and lower in different months/periods).
That can be more accurately summarized as player preference rather than the race’s skill requirements. Less people choose Zerg because they have aesthetic or preferential (preferred units, play-styles, etc) to pick Terran or Protoss over Zerg.
The skew of Zerg players towards middle/higher leagues can also be attributed to a lower skill floor rather than a higher one, since this would shift the players who do pick Zerg up out of the lowest leagues resulting in the skew present on that part of the ladder.