They are banking on first impression bias. They put in a lot of effort to get the audience to think "wow this is a great show. It's well-executed and I like these characters and want to see where it goes".
A lot of people drop off with the drop in quality, but my guess is that more people stay to continue watching than they would have if the budget has been so low from the start. In effect, the big budget of season 1 becomes more about marketing than it is about actually making art.