I would like to think that the reason my campaign isn't as successful as the Founders deck is because of the lack of extras like T-shirts and coins, but the data just doesn't support that. For Founders deck, there are 451 backers right now, but only 60 of them have pledged for anything that includes a T-shirt or a coin. That's only 13% of the total number of backers who wanted something extra. The remaining 87% of the backers are pledging for only card decks or card decks plus uncut sheets. That tells me that even if I added a shirt or a coin or poker chips or dice or whatever, I wouldn't see an increase in the number of backers of more than 10%-20%.
No, it all comes down to content and design. The content and design of the Founders deck has resonated with a large number of people, unlike the content of my deck, which is much more niche-focused. Hell, even I myself am thinking about pledging for a Founder's deck!