Part of it is deer densities, some is people densities. There's a larger population of deer west of I-35 in those areas than there is east of it.
There is also the effort by TPWD to 'improve' the deer taken from east of I-35 with the advent of the antler restrictions etc.
Part of all of this is due to the drought of the 1950's. For nearly 10 years the state went nearly without rain. The result was a near 100% die-off of whitetails in some areas. One study from A&M in Llano county suggested 95-97% of the deer herd in that area died due to starvation and lack of water. When the drought ended in '58-60 the herds started coming back.
Many that hunted in the areas west of I-35 will remember doe permits and those to the east may remember having 'doe days'. The result in a lot of the areas to the west of I-35 was/is an over population of deer, hence less restrictive limits.
These aren't the only reasons for the differences but they are some of the reasons.
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