While Chris Derrick was used as an example, I believe the decision not to go to 63 more to make the numbers of qualifiers similar, yet have most of the qualifiers come from the marathon, like the women. The number of men that ran 64:00 or faster was 41 compared to number of men that ran 63:00 or faster (13). Only 86 men qualified with a 2:19:00 or faster (14 made it "after the fact" when the IAAF relaxed the standard from 218 to 219). Using 64:00 or faster dropped 84 men that had hit the 65:00 or faster standard and would have yielded about 127 qualifiers in 2016 (you never know who else would have tried to lower their time to hit the 64 standard).
The women's standard of 73:00 only would have had 13 qualifiers (dropping 35 qualifiers with the 75 standard) and like Camille had pointed out that would have led to the same number of qualifiers from the half for men and women. However, 198 women qualified in the marathon with the 245:00 standard (39 of those women had made it "after the fact" when the IAAF standard was changed from 243 to 245). So using the new standards would have had 211 qualifiers for women in 2016.
Personally, I think I am in a similar boat as Camille in thinking that only a few people should be able to qualify from the half and the majority should be from the marathon. I would have definitely been for 220/245 but the idea with sticking with 219 from my understanding was just to make sure than in some extremely unlikely scenario if a 219:30 guy got top 3 at the trials, he wouldn't be eligible for the Olympics from not having the IAAF standard.
Also, to whoever asked about an American finishing top 10 at Boston in a really poor condition year and only have run 223, he would be an A standard individual per the IAAF Gold Label race standards and if he got top 3 and did not run under 219 at the trials, nor at any other race when you can get the standard for the 2020 marathon (how this would happen realistically, I do not know), he would be competing for the USA in the Olympics in 2020.