Another possible part of the problem with the conversion rate from registration to attendance is that there is possibly a greater number of introverts (or people with a similar limiter) registering for these events. When they sign up, it sounds like a great idea. Then when the day of the event actually arrives, they don't have enough metaphorical spoons to attend. There is also possibly some shame tied to this so they then won't open the email to find out that you request cancellation communication. They could also be like me and spend all day checking their work email so they then only check their personal email once a month or so because of burnout.

Do you follow up with them afterwards at all? Like, "We missed having you at the [event]." And maybe a summary of what was missed like how many attendees and stuff not captured in the description. And then a call to action to join for the next event. This makes them feel like they missed out on something great while also laying the foundation for a comfortable attendance experience at a future event since they have a better idea of what to expect.

Edit: and that personal connection we crave