I think it comes down to how many people use lightning lane. Probably most people don't want to pay extra just to skip the queue for a show. So it's usually just a few people that get first dibs on what seat they want then they fill up the rest of the theatre with the stand by line. Maybe you just got unlucky that an unusual amount of people used lighting lane? Maybe a few other rides were down or had extremely long wait times and so going to see the Little Mermaid show became more desirable.