Why? Maybe the market just isn't there for that many work management systems. Or maybe it's not a fun and exciting product to create, so a solo developer isn't as likely to pick it up.
Remember we're not talking about the general case here. We're specifically talking about the feasibility of seeing a specific product being shut down, and then building a clone of it on a small resource budget in an attempt to snatch up their soon-to-be-former customers.
