Why Do Options with the Same Strike Price but Different Expiration Dates Have Different Premiums?

Options with the same strike price but different expiration dates have different premiums primarily due to the difference in their extrinsic (time) value. The option with the longer time to expiration will have a higher extrinsic value because there is more time for the underlying asset's price to move favorably, increasing the probability of a profitable outcome.

All else being equal, the longer-dated option will always be more expensive than the shorter-dated option.