From what I see, the reasons for Iowa placing relatively high in recordbooks are summed up in the following:

Relatively low hunter dencities
No rifle, just shotguns which reduce range (excluding S Iowa antlerless the last couple years)
Genetics, but more so EXCELLENT FOOD & MINERAL SOURCES coupled with 'relatively' mild winters (Limestone country + corn and soy = well developed racks, period)
Gun season not during the rut
2 or 3 (if landowner) antlered deer per year per hunter

Note other than number three, the rest are directly controled by humans. Number three is a geographic fact. IA (principaly NE, SE, and Western portions of the state as everything else is flat as a pancake with little cover) allows a good number of young bucks to grow up to big bucks. Thats the secret folks. The IA DNR does a decent job of balancing farmers, DOT, and hunters intests in deer management. This means the vehicle colisions are at a manageable level, not too many farmers are complaing about too many deer, and not too many hunters are complaining about not enough deer. What works for IA would not work for PA or WI with deer and hunter populations 2 or three times the size.