The reason is simple: both have their inherent advantages.
Gloves are more traditional, comfortable to wear, protect your fingers well and allow you to do other things when you have them on. Moreover, they are quicker to use — there's no fumbling with them when game suddenly appears.
Shooting tabs, on the other hand, often provide a smoother release, allow you to hold a closer anchor point to your face, and can be worn over real gloves in cold weather. At least that's my experience.
I've used both over the years and have vacillated from one to the other, but for the last three years have settled on shooting gloves. I can't see that changing in the foreseeable future either.
For me, they are a more practical choice in the field as I like the way I can handle other items and nock an arrow with mine. Also, any loss of smoothness in release is made up for by the fact that, for some unknown reason, I simply shoot better with gloves. They just seem to be a natural extension of my hands.
More than likely, what you use depends on who taught you. And, if that's working for you, I wouldn't change a thing. But, if you are curious, haven't tried both and compared for yourself, this period between major seasons is the time to experiment.
You'll find there is a difference and one or the other will suit your style of shooting and comfort level far better. Even so, you might need to make subtle adjustments to get it exactly right.
With tabs or gloves you will likely have to experiment with size, fit and stiffness of leather or whatever material is being used. You'll also need to shoot each type for a couple of weeks to get used to the slight differences.
Once you've figured it out, my best advice to you is stick with it and then buy a back up of whatever it is you use. (Gloves develop string grooves, tabs sometimes break; both get lost or, as I found out once, taste good to a dog.) Shoot with your back up too so it is broken in when and if you need it.
In the end, this is really about personal preference and confidence in what you use. Pick one, practice hard and know its nuances. Bow season will be here sooner than you think.