Polyurethane is cheaper. It's a one-step finish. Nitrocellulose lacquer has to be buffed out in many coats. So the sound would change again on most guitars. Now, I've seen polyurethane guitars that get it done, but on a Les Paul, you can hear the difference a lot of times. The polyurethane is a harder finish. So you may get a little more sustain because it's resisting absorption. Your string will sustain longer, but resonance decreases. Sustain is not resonance. It's not even frequency response. It's just dumb sustain. So the urethane made a difference.
Most standard guitar brands and most guitars being made now use urethane. It's much less hassle, easier to work with, and dries relatively quickly. It can be completed in a couple of weeks. Nitrocellulose finishes done properly take months to cure. They feel dry within a few weeks, but, it takes months for the finish to harden. Guitar companies using nitrocellulose paint either have large work areas and curing space or tend to charge more because of how long it will take the guitar to be ready for shipment. It just slows the build process way down.