Ask HN: Why do so many web startups use subscription based revenue models?

This came to my attention when I read about Amazon Polly on HN. I wanted to convert a long text into an audiobook. A user commented on that Polly thread offering his webservice which for $ per month subscription allows doing this. However, On AWS I'd pay per word; why don't more companies adopt a pay for usage model? I didn't subscribe to his service because I only have 1 text to convert, not an ongoing need.