It's likely a combination of a few things including people going directly to NFL (Gamepass), NBA, NHL and MLB for the content as well as people simply dropping cable altogether. Those cable subscribers might not have cared/watched sports anyway, though they /paid/ for it and were counted as "subscribers" on ESPN numbers.

Another way to think about it. ESPN, outside commentary and opinion, is really nothing more than a middleman. The leagues can go direct to the consumer and they likely will do this much more in the future.

ESPN cannot go direct to consumer per the contracts. It is required that to consume much of the ESPN content, you need a cable subscription (even to stream on the internet). That is a broken model and is slowly (or quickly?) dying. That's not great for NFL etc but horrible for ESPN. The sports leagues could recover, ESPN likely couldn't.