I guess to stop players of popular games getting spammed?
Invitations are not the only way of filling a tournament. If it gets popular enough, it will show up high in the list of tournaments all players see. With 40 invitations to well-chosen recipients who are likely to join, your tournament will get popular enough to be quite visible.
You can also paste the link to your tournament in the chat for Dobble games you have played, but I would make sure you've talked to the other players first and asked them if anyone objects to your sharing the link there. That way no-one will feel you are using the game to spam them.
