Why are audio power supplies center negative?
It simply is what it is.  There is no reason to expect either polarity.  Probably someone early in this market picked center-negative arbitrarily, and others followed the trend.

It's a arbitrary choice, so neither is more wrong or right than the other.

You should, of course, never expect any particular piece of equipment to use a particular orientation or be compatible with a particular power supply unless it says so explicitly or the power supply comes with the product.