Why is Callisto's potential subsurface ocean considered less favorable for life compared to Europa's?

Answer

It lacks contact with rocky material.

Callisto's potential ocean is considered less favorable for life than Europa’s because it lacks contact with rocky material and is heated only by residual radioactive decay rather than strong tidal flexing.

Why is Callisto's potential subsurface ocean considered less favorable for life compared to Europa's?
SpaceJupitercolonizationmoonsCallisto