Leftists believe diversity is important inasmuch as it disempowers those who they think are unjustly at the top of the socioeconomic hierarchy, that is people of european ancestry.
They couldn't care less about diversity qua diversity, in fact you'll often hear them praise a hypothetical future in which the whole human species is so thoroughly mixed that you can't tell anyone's ethnicity by looking at them, in other words a world with the lowest amount of diversity possible.

Anyone who actually cares about diversity immediately realizes that the only way in which you can guarantee the presence of a great deal of variety in the world is to have different areas for different people.

Now, it's not that leftists are stupid or that they don't know this, it's just that it's a political matter so their brains shut down as soon as you talk about it. They understand pretty much every identitarian talking point as long as it's not about ancestry in humans.

If you tell them that a certain subspecies of rhinos that it's not even clear if it's actually a subspecies or not but it's peculiar enough to stand out, is probably going to die out because of, say, climate change pushing other rhinos in their areas, they immediately mobilize to try to preserve those handful of rhinos: not even one of them goes "but who cares about genes" or "why does it matter if those rhinos die out, there are still going to be other rhinos around!". Not a single one of them says anything of the sort, in fact, those are probably the parodies they're going to create when they satirize the "conservatives" who don't care about the preservation of nature.