Just to be clear,
force:createRecord does not create a record.
This event is handled by the one.app container. It’s supported in Lightning Experience, the Salesforce app, and Lightning communities. This event presents a standard page to create a record. That is, it doesn’t respect overrides on the object’s create action.
It presents a standard record create pop-up, pre-populated by the indicated values, and it's up to the user to fill in the form and click Save. Then a record is created. If you're not getting the pop-up in your Lightning Community that's one issue. If you're getting the pop-up and clicking Save and the record gets created but no assignment rules are triggered that's another issue. Which is it?
If your intended behaviour was just to create the record without user intervention, look into Lightning Data Service.