Because some surgeons believe that the pressure of the seat nose can impede erectile recovery. As my husband's urologist put it "what's the point of me going to all that effort to save your nerve bundles if you don't give your nerve bundles the best chance of repairing? I may as well just do non nerve-sparing." In our case, the uro asked him not to get back on his bike for 6 months and when, he did, we bought prostate friendly saddles for his favourite bikes. Once the idea is in your head, it feels foolish to ignore such a strong view from the expert.
Having said that, there are plenty of urologists out there who don't seem to believe that cycling will make any difference to recovery of erections. Each man has to just go by what his specialist says and then his own research and approach to risk.