Exactly!
Incidentally, I designed a graphics for my daughter just 2 hours back in PS... she selected two Disney images, I composed them into one and did all that was needed to do (clearing the BG, resizing, modifying certain things like lighting, levels, resizing, resolution, ... added a few things, and then exported as PNG. I uploaded to a site where a women's t-shirt was ready to take the design. It asked a few things, and it was ready. I gave the necessary details and paid through online banking... All that process was completed in less than 2 hours -- from asking her what design she wanted, then spending some time googling the images, then designing, etc. .. up to the completion of the payment transaction on the site.
I will get the t-shirt in not more than 4-5 weekdays! No waiting for a set number of orders to be booked etc... this is how we get things on ebay and amazon. You select and order, and they give you acknowledgement of the payment. All that happens in 10-15 minutes max! The only additional step involved is the custom designing part... And with today's advanced garment printing technology, there is no need to create screen-printing mesh etc. I am sure that TeeSpring also does the DTG (Direct to Garment) digital printing and not the old-fashioned screen-printing processes. So why do they need minimum 3 orders to set up a transaction?! Are they aware that they are losing, I guess, almost 90%+ of the business because of the needless wait for 3 customers' orders...?
Also the cost of the t-shirt on that site is about 50-100% more than what you'd see on a normal marketplace. $19.95 for a simple t-shirt with about 3 lines of text is too much imo. If they do the business like what I have experienced above, then they can reduce the cost, and still they would earn profits along with the designer because the sales would shoot straight up imo as the only hurdle, that long delay (waiting for 3 completed orders), is removed.
I think this is it. It's what I was thinking even before you wrote it. Nobody wants to give away details and make a payment that hangs in limbo. I wouldn't.