Advertisers lost interest, so there's no resources to promote some random podcast. If you listen to something like "Coder Radio" from Jupiter Networks you will hear the hosts talk about there being a lack of ads and Leo Laportes TWIT network is expressing the same concerns. Both of those two podcast producers will survive, because their audience is willing to pay for the content, and because they have a few niche podcasts where advertisers are able to target a narrow group of people with the right messages.

Large podcast aggregators like Spotify have no real business model, they produce way to many shows with to fewer advertisers and subscribers to cover the cost and their audience isn't nearly as loyal those of TWIT.

Partly I believe that there's simply to many podcasts available, with to low content quality.