I have done tests where I put areas of just craft paper down in the enclosures and other areas of loose susbstrate (aspen or cypress) and have found (after many different test configurations to eliminate the chance of habit, security, or temperature playing a role in snake choices) that my ball and my boa both prefer loose substrate over flat paper. The boa especially likes to burrow in the substrate. Both definitely choose the loose substrate to lay on rather than the paper. I won't speculate on why but that's how it stacks up in my house.