Why does slim fit not fit?

I came up with TempleTribe for men who workout and are sick of paying for top brands that don't fit or feel like they should. I wanted a product that was cheap enough for a college student yet was tailored fit, anti-sweat, and anti-ultraviolet. So i created my own.
I have always been a fit guy and have worn slim fit shirts. My shoulders were always too tight and I got horrible, and I mean horrible, pit stains. So I designed a shirt that would be great for athletic men. This is where the idea of TempleTribe came from.