In quantum mechanics, the phenomenon you are referring to is known as the "collapse of the wave function" or "wave function collapse" after measurement. When a quantum system is in a superposition of multiple states and a measurement is made, it is generally observed to be in only one of those states. This observation is known as the "collapse" of the wave function to a specific eigenstate.
The reason for this collapse is still a subject of debate and interpretation in quantum mechanics. There are different interpretations of quantum mechanics, such as the Copenhagen interpretation, the many-worlds interpretation, and the pilot-wave theory, among others. Each interpretation provides a different perspective on the nature of the collapse.
In the Copenhagen interpretation, which is one of the most widely accepted interpretations, the collapse of the wave function is considered a fundamental and intrinsic aspect of the measurement process. According to this interpretation, before measurement, the system is in a superposition of all possible states, and the act of measurement causes an instantaneous and random "jump" to one of the eigenstates of the observable being measured. The probability of obtaining a particular eigenstate is determined by the squared magnitude of the corresponding coefficient in the superposition.
The other states that were present in the superposition after the measurement are no longer observed or accessible. They are said to be "discarded" or "irrelevant" for the specific measurement outcome that was obtained. These unobserved states may still exist in a superposition but cannot be detected or interact with the observer or other systems in a way that reveals their presence.
It's important to note that the different interpretations of quantum mechanics offer different explanations and philosophical viewpoints on the nature of the wave function collapse. The collapse itself is a mathematical description of the probabilistic behavior observed in experiments, but the underlying mechanism or physical process that leads to the collapse is still an active area of research and debate.