Cyclic hemiacetals are more stable than acyclic hemiacetals primarily due to the ring strain being relieved when forming a five or six-membered ring. These ring sizes are energetically favorable because they reduce the torsional strain and allow the atoms to adopt a near-ideal tetrahedral geometry. Furthermore, intramolecular reactions, which lead to the formation of cyclic hemiacetals, are more favorable compared to intermolecular reactions because they benefit from the "chelation effect," where a molecule forms two or more bonds to the same metal ion, stabilizing the structure.