What explains the poor development of the sense organs in roundworms?

The sense organs are the organs necessary to adapt to the living conditions. In particular, they can be used for orientation in the environment for the purpose of obtaining food. Since roundworms (for example, the human roundworm) lead a parasitic lifestyle in the intestines of humans, they no longer need to search for food. In other words, food is supplied to the parasite by its host directly. And therefore, it also disappears to adapt to the conditions of its habitat.
Note that not all roundworms have no sense organs. In free-living representatives, these organs develop poorly.