Welcome to the Atlassian Community!
Because you have signed up to use Trello (or another Atlassian Cloud-based system). Atlassian Cloud systems have 2-3 account systems, and you have chosen to use one of their systems that need it.
Atlassian Accounts like the one you used to post here are probably the most widely used, but your organisation may be using Atlassian Access to provision accounts, possibly from other directory systems. The "2-3" bit comes from the ability to use Google Accounts as well (which sort of create an associated Atlassian account that you do not need to think about)
Atlassian accounts can be used to log into all Atlassian Cloud systems, but they do need the owner of the system to let them in. In the case of Community, here, it is intended to be available for anyone who might be using Atlassian stuff, so it is completely open. If you have an Atlassian account, you can use Community.
TLDR: you, or one of your admins, has signed up for your account.
You must be a registered user to add a comment. If you've already registered, sign in. Otherwise, register and sign in.