That might be a localized phenomenon I reckon. I split my time between Arizona and Wisconsin and in both places I ski, slalom is alive and well. 6 years ago when I got to Arizona, there were maybe 2-3 boats on the public lake I go to that were slalom skiers. That's at least doubled since. It's a small crew, but they're still out there, including a couple newer Nautiques and ProStars.
In Wisconsin, both in the northwoods and in the southern lake country between Madison and Milwaukee, morning slalom is thriving.
I know this site is mostly focused on private lake crews turning buoys and shortening the rope, but there are thousands of us out there who ALSO or ONLY open water slalom, and my family has skiers as young as 10 and as old as 75 going out in the morning together. In our Malibu.