I contacted the race organisers regarding the route selection for stage 3 because, to be honest, I think it's a real shame that the stage will probably end in a boring sprint, despite the potential that the region has to offer. When Kassel was chosen as the destination, the original plan was to ride the Essigberg (300 metres of elevation gain) in the west of Kassel shortly before the finish.
Here is the translated version, what the race organizer responded to me:
"We had actually considered entering Kassel via the road you mentioned in Druseltal/Ehlener Straße. However, there is a tram turning loop in the city area with tracks running parallel to the road, which unfortunately made this passage impossible. As much as we would like to take in every hill in the region (there are still a few north of Kassel), there are limiting factors such as motorway exits and railway crossings that we have to take into account. A final lap in Kassel was also planned at one point, which would certainly have made the whole thing even more exciting – but unfortunately this could not be implemented for organisational reasons."