There's 2 main reasons:
1. ONE-SIZE-FITS-ALL REQUIREMENT
Creating a one size fits all strap is a major challenge, but was an important criterion for us in designing Slide and Slide Lite. The best thing about designing something that’s one size fits all is the fact that consumers don’t have to make a choice when deciding which one to buy. We sell a lot of gear via the ol' Interwebs, where size choices are extra-difficult to make.
2. UPPER-BACK TO LOW-TORSO CARRYING HEIGHTS
Slide is designed to carry a camera in a nearly endless array of options, from tightly against your back and high up (for moving around) to low on the torso at extended arm length (so you can grab your camera and shoot). Slide is not intended to carry your camera at or below the hip - this was deprioritized due to a combination of customer feedback and our own thoughts on what are safe, comfortable ways to carry a camera.
We wanted to ensure that we could accommodate these intended uses across a wide range of body types. On the short end, we wanted a strap that could cinch up on a short person with a narrow torso. On the long end, we Slide to be able to carry a camera in the small of the back for a tall person with a large upper body.
WHAT IF SLIDE IS TOO BIG OR SMALL FOR ME?
Our first suggestion is to try to wear it as recommended: Behind the back, about even with the right kidney (for those of us lucky enough to have two).
The reason we recommend wearing it in that location is because it makes the product truly function as a no-handed strap. You can place your camera behind you and comfortably walk around with both hands free, and your camera will actually stay in place without flopping or swinging around. No other strap allows for this.
If that simply doesn’t work for you, we want to hear. We've considered making a Slide XL for folks with larger frames. Shoot us some feedback here and tell us more about what you'd like to see.