as someone who has hunted in the hill country (bandera mainly) and east texas (tyler county) for years, the question of why is there so many more deer in the hill country than east texas has always came to mind and i could never think of a good reason. East texas has plenty of room, natural vegetation and just about everything else deer need to thrive so why is there such a difference in the numbers between the two places?

Lack of predators was a huge reason Hill Country numbers were always high. Timely rains and mast crops keep the deer alive. Just because it is green does not mean it is good for deer.

I grew up in the hill country around Bandera and I didn't know what a Coyote sounded like or even saw one.
Only one I saw was one killed by a trapper and that was a big deal when I was a kid.
The Sheep and Goat ranchers used to make sure they kept them in check.
Also the deer herd exploded when the Screw Worm Sterile flies were dropped over the hill country and eradicated the screw worm back in the sixties.
I used to find those empty cardboard boxes with the red bullseye and explination printed on the box all over the ranch that I used to roam as a kid.
Thanks to development, and ranches being sold and subdivided, and the sheep and goat business going away years ago, predators are gaining a foothold in many parts of the hill country, and I know of several ranches with low deer populations and high predator numbers as well.
Other factors figure in such as drought and hunting pressure, but the deer thrive where there is plenty to eat.