Linked from
The 20 pages that link to Grand Unified Theory, each with the reason it gives.
Magnetic monopoleRelated: Many such theories predict monopoles formed in the early universe.
Lepton numberRelated: Many unified models allow processes that violate lepton number.
Electroweak theoryCompared with: It seeks a broader unification than electroweak theory achieves.
B − LRelated: Many grand unified models place B − L within a larger gauge structure.