Podcasting is drying up because the money left. Everyone went all in on podcasts on 2020. Spotify bet the farm on podcasts. Money poured in. Marketing bros realized there's only so many mattresses and underwear you can sell through the format and left.
You really can't serve personalized ads through podcasts. The relevance of what you advertise can be about the topic of the show (that is, marketing to the type of people who would listen) or the location of the listener. Pretty much every other signal gives you nothing interesting you'd be about to decide "yeah they're a potential good customer". Spray and pray.
The money left. People realized they couldn't justify the time and money they pour into podcasting. It turns out, even if you weren't expecting to make money, you really hoped people would listen. Not enough, because podcasts faded and people discovered TikTok. No more waiting for your favorite show to drop: everything is your favorite show. If you get bored just scroll up.
Lots of folks are still making it work. But a lot more people are going into podcasting with a more deliberate approach. People are doing it because they think it's important, not because they think people will listen or because they want to get rich. I'd argue that some of the best podcasts ever made have come out in the past 2-3 years, but if you're not giving the median listener the thrill of the first season of Serial, they don't listen past the first episode or two.