Because there are so many users visiting our server everyday. In order to reduce the web traffic load so that everybody can access the resource without too much delay. We ask user to wait at least 3 minutes for the next request. It should be enough for normal use and we believe user should at least read the message or play with the simulation for a while before user request to download it.
Actually, we have found many users request our server to send too many simulations to their email account. The mail quota was reached before we send all the simulations. And the email was bounced back to our server and increased the server load a lot. (Especially for simulations created with EJS -- The file size of EJS libraries are near 2M.)