Why not? Because it is not maintained and things move on regardless of how stable it is today . The core code may be extremely stable / bug free but server software is being updated regularly, API changes are being made by companies out of AWBS's control such as Payza and Skrill, Nominet etc
Sooner or later you will have to move to another system (unless they actually start supporting the project again) so if you're running a business, it might be worth investing into having a migration module built whilst your business is still working.
You could play with a double migration though. AWBS -> WHMCS -> Blesta. I'd be sure to thoroughly test it though as things may get messy / break