Not a lot of people know, but the Van Allen "belts" are not continuous around the Earth. There are sections that are more intense than others, there are gaps and odd-shaped zones. Some of the regions are transient, and dissipate and re-form. The first penetration of the Van Allen regions by a manned spacecraft was Gemini 10, which made repeated traverses of the Southern Atlantic Magnetic Anomaly, which is a low-hanging swath of the inner Van Allen belts.
That makes sense that some regions of the belts are less intense and that they change, dissipate and reform over time. But its said Apollo 11 was planned so as to go around the these more intense areas. So why cant they do that know? (They literally say they can’t accomplish it now). Also, I don’t understand what the duration of today’s missions have to do with it since no mission would spend any longer time in the belts than necessary.