According to the theory of decoherence, the interaction of a quantum system with its environment leads to the loss of quantum coherence and the emergence of classical behavior. In the case of subatomic particles in a "regular" solid on Earth, their interaction with the surrounding environment, such as other particles and the thermal energy of the solid, causes rapid decoherence. This decoherence effectively destroys the delicate quantum interference effects necessary for quantum tunneling to occur over macroscopic distances.
In contrast, subatomic particles in the cores of stars experience different environmental conditions. The extreme temperatures and densities in stellar cores create an environment where quantum tunneling can play a significant role. The high energy densities and strong interactions between particles allow for the existence of conditions where quantum tunneling processes can occur over macroscopic distances.
It's important to note that the exact conditions necessary for quantum tunneling to occur depend on various factors, including the energy barriers involved, particle properties, and environmental interactions. The theory of decoherence provides a framework for understanding the transition from quantum behavior to classical behavior but doesn't directly address the specific conditions for quantum tunneling in different systems. The study of quantum tunneling involves a combination of quantum mechanics, statistical physics, and specific system considerations.