You seem more relaxed, comfortable, genuine, and are at least low maintenance enough that you can go out in public(presumably to work out) without full makeup.
A girl that will never leave the house without full makeup, especially the gym, is a red flag for a lot of men; because high maintenance and insecure.
Make-up is usually too noticeable when I see most women put it on, it just feels fake seems too “prissy”. As a guy, I prefer “natural”, no make-up, it just looks better. In my mind, athleisure clothing is just generally more form-fitting and more likely to show off your body, but that can be a double-edged sword if you don’t have a body that is flattered by that kind of clothing. Just guesses as a guy.
As far as the athleisure is concerned, that should be self evident. Simple easy to remove clothing that lets you see what you're getting. (A crude description but accurate)
Makeup is a hassle, as are women who are very appearance focused.
The simplest answer is that you look better like that.
A more likely answer is that you look more like what guys are looking for like that.
The first time I hit on my now-wife, she just had prominent eyeliner on, and I dig that. I've never been big into the full-face make-up thing, and lipstick is hit-or-miss depending on the color.
The only time most men consciously register makeup is when you're not wearing any and they say "you look tired." But the athleisure is probably brightly colored and makes you seem approachable instead of headed to an event.