In gymnosperms, one sperm degenerates because
Gymnosperm ovule typically contains several archegonia each with egg although usually only one becomes fertilized; pollen tube delivers two sperm, one fuses egg, second degenerates due to absence complementary counterpart and lack endosperm sink formation as in angiosperms. Cleavage polyembryony common produces several embryos per seed via zygote splitting early mitosis, intense competition among embryos within female gametophyte ensures only most vigorous survives while others abort nutrients recycled. Degeneration reflects that only one egg functionally matures per archegonium and allocation favors single dominant embryo preventing wasteful multiple seedlings competing within single seed with limited food reserve.
Ref: Campbell Biology, 12th ed., Chapter 30: Gymnosperm polyembryony single embryo dominance degeneration mechanism