Why did this just happen ?

I think it’s the sampling rate, dropped bits, or elimination of noise on the USB. PCM uses packets to wrap up and send information. If it runs out of time, it drops bits. The cost is resolution. Computers are getting better and better at this. It’s why I went to DSD back in the day. 
 

Full disclosure - I’m an analog guy. I do use a PC for my digital (the horror) but I do not stream. Everything I listen to I have spent the past 25 years recording and cataloging myself from analog sources. For how I use it, it’s fine for me. Say what you want to say about PC/Mac as a source, I don’t have enough years left on this planet to redo that work. You didn’t say if you were streaming or playing from your drive. If you’re going to consider a streamer and you have cataloged your songs, I would look very hard at the interface and how it could hold onto that information. 
 

Regardless, this sounds like a good problem to have. Congrats and enjoy. 

Less noise

You didn’t specify which MacBook Pro you are now using. If it is one with the M1 processor, that is part of the answer, plus better/faster processing from the ram, storage, and input and output. Beyond that, results will vary depending on what else is connected to the Mac, and what other software is running. Changing the USB output cable to accommodate the USB C likely also contributed some to the change. Speaking from some experience going from an old Mac Mini 2011 to a new M2 Mac Mini 2023. Even with the newer processor which stepped sound quality up, a new better USB cable also made a difference.

@eugenegearty

Always refreshing to hear good news when new "stuff" is swapped out and something positive happens.

I’m with @budjoe on this one. Changing to USB C indicated a definite change in cabling. The "sonics" of cable changes can range from "whatever" to precisely the phenomena you described -- "wonderful improvement."

What to do next? Staying the current course, maybe get your hands on some (really good) USB C cables for a listen? Taking it to another level, perhaps try a dedicated (real) streamer? Or, as @asvjerry suggested, contact a computer guru (or become one yourself) and build a box that can rapidly calculate the Drake Equation as well as spit music out the back of the box.

Enjoy.

What dinov said, less noise.  You went from a noisy old iMac with a built-in AC-powered switching supply and a big ol' screen, to a laptop with a DC supply powered from an external source and a much smaller screen.  You also upgraded the processor, memory, etc.  As others have said, now try a real streamer.  It doesn't have to be terribly expensive, about the same as the Macbook, but it will be even quieter and more resolving.