It's a de facto standard because markets for common stock generally don't exist for private companies. If you're comparing Uber against AirBnb it makes sense to

It's a de facto standard because markets for common stock generally don't exist for private companies. If you're comparing Uber against AirBnb it makes sense to compare valuations based on preferred stock, if you're comparing them to anything in "the real economy" or are interested in the value someone has placed on the company as a whole it's less clear which number is more useful.