Everyone loves free t-shirts at tech conferences, so why aren't more given out?
I'm not a billboard.
When I go to a conference, I don't want to wear a shirt for a product or company unless I'm fully representing them. This means I work for them or I'm really passionate about them... AND they look good on me (e.g., EFF hoodies).
Shirt swag at a conference seldom ticks those boxes.
Socks, on the other hand, are much easier to do when I'm not super into it. I have swag socks for companies that don't exist anymore and will still sport them. They're not as obvious as a tee.
Try more socks!