Nothing in particular, it’s an arbitrary starting point. But that’s really not a good reason to knock it.
Does water actually freeze at 0 celsius? It depends on the air pressure, right? I guess 0 celsius is the freezing point of water at sea level, but air pressure’s not consistent at all. I guess maybe it’s the temperature water freezes at the average air pressure at sea level? I assume that’s the case.
The point I’m trying to make is the Celsius isn’t super rock solid either, and it really doesn’t affect anything if water freezes at 0 or 32 degrees. The best argument for celsius is that it’s standard, but that doesn’t make necessarily make it better.
If we really cared about having a rock-solid starting point, we’d use Kelvin because you literally cannot go below 0.