tl;dr

Mistake 1. They have collected beta invitation email addresses but did not send any emails until the launch. Some users forgot about the product when they received the invitation 3-6 months later.

Mistake 2. They used spreadsheets to track user cohorts and that was too time consuming.

Mistake 3. They focused too much on features instead of benefits.

Mistake 4. They weren't able to find ways to get exposure from blogs.