I see that power supplies for guitar pedals, speakers, etc are always center negative. Is this a conscious design choice, or is it a marketing trap?
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.