That is because a podcast is just an rss feed of audio files… to get that sort of information (from all your listeners) you have to add to rss and break compatibility.
I think we have the capability in RSS since the age of blogs and RSS aggregators.
Basically the aggregator can grab the main RSS feed and tell the server "I'm not just one person, I'm actually serving this to {subscriber count} people" and the server can use that information if it wants to.
Basically the aggregator can grab the main RSS feed and tell the server "I'm not just one person, I'm actually serving this to {subscriber count} people" and the server can use that information if it wants to.