If a cyclopentane has two substituents, one on carbon 1 and another on carbon 4, what is the primary reason for the preference of numbering the ring starting from carbon 1 instead of carbon 2?

When you number a cycloalkane, you start at the carbon that gives the lowest possible set of locants for the substituents. In this case, numbering from carbon 1 places the substituents at positions 1 and 4, giving the locant set 1 and 4. If you began numbering at carbon 2, the same substituents would appear at positions 2 and 5, a higher set of numbers. Since the set 1, 4 is lower than 2, 5, the IUPAC rule of “lowest locants” dictates that the ring be numbered from carbon 1. For example, a 1,4‑dichlorocyclopentane is correctly written with locants 1 and 4, not 2 and 5.