Official comment

The official answer is that this mixer is primarily going to be used with a computer, so it's easy to update the settings. For those situations where the mixer is installed in a venue used by multiple DJs (hopefully that will return soon!), having the mixer return to the defaults between DJs so they know what to expect instead of encountering weird customizations, was preferable.
We'll pass this feedback to our product team for future consideration. Thanks for your feedback.