It's both genetics and nutrition, and it's too complex to simply break it down into something as simple as minerals or ag crops or browse.
You bet big bucks where you CONSISTENTLY have good nutrition and good genetics are present. Nutrition has dozens of variables as there are many different food sources for deer in different areas of the country. Now, good genetics are present in every population, however, they will be more greatly expressed when there is consistently good nutrition. Poor nutrition often results in does producing fawns with lesser genetic potential. If this happens in a recurring manner, then the over genetics of the area will decline.
I've kind of summarized some reading I did at this site. I'd check it out if you're interested in more info.
http://www.tpwd.state.tx.us/huntwild..._wma/research/