I don't know about Trello, but i understand why B2B SaaS have this problem.
The main issue is the ability to support various authentication systems, protocols, eg. Okta SSO, Google, Password, Mobile apps, Custom SAML and dozens of other enterprise-y stuff.
To be able to support that most apps do it as first step is ask for an email to be able to redirect them to the right flow. So the problem is bootstrapping how does a user confirm it's him before he can login to the right system.
Most B2B apps are forced to deal with this because there's no one protocol here, and different paying customers have different internal systems. Asking the user to choose from a dropdown of 20+ paths is proving to be impossible of the extremely high customer support costs.
To be able to support that most apps do it as first step is ask for an email to be able to redirect them to the right flow. So the problem is bootstrapping how does a user confirm it's him before he can login to the right system.
Most B2B apps are forced to deal with this because there's no one protocol here, and different paying customers have different internal systems. Asking the user to choose from a dropdown of 20+ paths is proving to be impossible of the extremely high customer support costs.
It's a cycle of misery