What specific feature makes Europa a primary target in the search for potential life?

It likely holds a subsurface saltwater ocean in contact with a rocky seafloor.

The scientific interest in Europa stems largely from the theory that beneath its fractured and chaotic icy shell lies a vast liquid saltwater ocean. This potential environment is considered highly significant because the liquid water is hypothesized to be in direct contact with a rocky seafloor. On Earth, similar interactions at the bottom of the ocean, such as those found at hydrothermal vents, provide the chemical energy necessary to support diverse biological ecosystems. Consequently, this interface is viewed as a prime candidate for finding life, as it could supply the essential chemical ingredients required for life to flourish even in the absence of sunlight.