Here are my two cents. Correct me if I am wrong.
Typically, n-sub and p-sub doping levels are close to each other. The doping level of the well needs to be at least one order higher due to the diffusion process. Compared to an NMOS in a p-sub technology , an NMOS in a n-sub technology requires a p-well as the bulk. The high doping level in the p-well bulk brings some undesired effects, for example, higher threshold voltage.