My opinion is that it is not profitable for any company to come with Linux t-shirts. Since Linux has a small share worldwide and Windows a hugh share, not many people would know what Linux is. So, for manufacturers there's not a large market for selling Linux shirts.
If you want a t-shirt or sweater or other stuff with the Linux Mint logo, then this would be put forward by the community to find a manufacturer which can, in limited quantities and at a low price, produce clothing with the Mint logo and use the profits for supporting the further development of Mint.
An other option is to print the t-shirts at your own cost at some print store.