Because it takes some time to adapt to the new level of light in the environment
Explanation:
The pupil of our eye functions to control the lightthat enters our eye by contracting or relaxing.
When we are in bright place, the pupil contracts to limit the light in our eye lens.
When we move to a dark room, the pupil takes some time to expand and due to this only limited amount of light enters our eye lens which might not be enough to see properly in the dark place.
Unlike human beings some animals have more effective pupil with which they can stalk their prey in low light too.
