I don't know if that was the purpose or just a side effect, but I would have thought that the purpose of a bypass is to provide a more attractive route than the one going through the town center (so motorists have an incentive to use it), and having to slow down at the roundabouts (along with the mentioned tight curves and the 60 km/h speed limit) completely destroys the attractivity.
drivers may interpret it differently, but flow is typically faster than stop&go, plus the illusion of "if you're moving you're getting somewhere" helps.
