Recent DIII grad, multiple time AA both track and xc.
The question isn't DI/DIII. There is plenty of variation among all of this. There are DI programs that are athletically intense and academically less intense, there are DIII programs that are academically intense and athletically a joke, etc. Every possible combination of the 3 - DI/DIII, Academically intense/weak, Athletically intense/weak (although I knowledge DI has a potential to reach extreme levels of athletic intensity). That still doesn't prevent fantastic coaches, better than most DI ones, from existing on the DIII level . Perhaps they like it on the DIII level because you get guys who are in it "for the sport" instead of because they need the financial support or needed to go to a big name, etc. It all depends on what kind of person the athlete is. I think if you have the potential to be a NS or WL, you're gonna be able to make it almost any way you please, be it DI, DII, DIII, NAIA, etc.
I personally looked at DI and DIII schools pretty equally, emphasis appropriately on "I am going here to get a good education, athletics is an important part of my life, so I want to be able to run there, but it is not going to be the real focus (although now I am certainly dreaming/working towards running at a professional capacity, didn't know I was going to get to that level at the time) I was a sub 9:30/sub 4:30/sub 16:00 5k XC guy at the time I was looking around. (SR year track ended up running times that probably would have gotten me recruited to a middle of the pack DI school if I had run them a year or so earlier.) So nothing of interest to most D1 programs, although I was trying to swing my consistent and gradual improvement over the years to make myself appear better. Times like that are still pretty good, you're not going to find a single DIII coach out there who doesn't think that someone who ran sub 9:30 as a JR could be an extremely valuable part of their team in the future. I ended up choosing the DIII school I went to because 1. It was academically very good. 2. The environment was not nearly as cutthroat/intense as some of the other schools I was looking at (UC Berkeley, a couple Ivies, etc.) which was something important to me because while I was a good student, I wasn't the best student, and was worried without a good support network, I would be miserable/failing and 3. I really liked the coach.
I think DIII was good for me partially because of the particular academic atmosphere I was in. It was rigorous, but not overwhelming, and coexisted well with my ability to train and also live a normal college life (have friends, party occasionally, be generally happy.) I think if I had gone to a DI school I would not have had that same school/athletics/social life balance. Potentially could have been slightly better athletically, sure, but worse off overall. If I had to do it over again, I still would still choose DIII. DI was I think too much of a mystery/risk/crapshoot for me. I would not be surprised if this is a common thread among some of the more talented kids who go DIII these days, but I'm sure theres a plethora of reasons.