On the forum, I can log in with username / email address and a password.
But on the shop, I can no longer do so. Instead, it wants to email me a six digit passcode.
Email is not instant. It is especially not instant if your email server is a bit fussy about whose email it will accept on first go and who it asks to try again in a bit.
Passcodes are also more work for me. Instead of going ‘just paste it, password manager’, I have to look at my email client, collect new email, find the email, copy the code, and then paste it.
Why stop even the option of using passwords??