Stackless is missing some important parts. I've spent a great deal of time working with it over the past few months, and there's no good SSL socket support. The built-in socket module will block the entire process (all tasklets), and while drop-in replacements exist, they either don't work with SSL, are buggy, or both.

Stackless also lacks Erlang's transparent support for multi-process and multi-machine execution, though I talked to a guy who's working on adding that for his Master's degree.

I'll be the first to admit that http socket issues exist and drop-in replacements are indeed often buggy. That being said, this wouldn't be a huge issue to overcome were Stackless given more TLC. Regardless, it has many other valuable uses.