Why did this just happen ?

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.

The MacBook Pro has the M1 chip. As you can tell I don't buy new stuff, in fact my bought used Koetsu Rosewood cartridge has been re-tipped 3 X. Crazier than that I have a Well Tempered Turn Table. So pretty much all my gear is from last century. The good news is it sounds REALLY good.My Benchmark USB DAC was an up grade from everything Audio Alchemy made. CD transport/DAC/anti jitter clock...Which was an upgrade from a Mod Squad Marantz CD player circa 1987.

I mainly play my records ,though a good digital front end is handy.

I am not interested in streaming in any form. My 2TB SSD has thousands of my CDs that I ripped over the years at the highest bit rate and AIFF format. The many HD tracks that I DL @96k also sound very good. So using a Mac and iTunes as my interface is not going to change. I was just amazed that a $400.00 upgrade in a PC made such a difference . The USB C output goes to a USB B hub so I can get to my Benchmark DAC.

Thought you geniuses had a detailed answer. No problem I kinda don't even want to know. My ears tell me everything.

Thanks