Democracy fundamentally depends on compromise and consensus to function. We reach these things because we have shared values and shared values generally lead to shared goals. You can have diversity of opinions and ideas but all of those things are just disagreements on how to get to the end goal; not a disagreement on what the end goal itself is.
But the more different types of people you inject into the mainstream of the same social structure the more different values you get. the more different histories, cultures and goals all compete for the mainstream.
Eventually compromise or consensus become impossible and you are forced to choose....Democracy or Diversity? You can't have both forever