Yes, I think it's because Substack is expanding exponentially. More people are joining and coming across your Newsletter.
Folks are conditioned to ‘follow’ they don’t know that they need to subscribe to get your content direct. I have been puzzling how to overcome the same thing.
When a couple people said that they were looking forward to getting my articles/podcast - I explained that they need to subscribe and they did. 🤷♀️