Question

Why might someone choose to use U-238 over C-14 to determine the age of a specimen?

Answer 1

Carbon 14 and and Uranium 238 are not used together to determine fossil ages, because of the huge differences in the half lives of C -14 and U -238 that cannot be used together. C -14 can only be used to date fossils of a very recent age. U -238 can only be used to date volcanic rocks of a very old age.
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Answer 2

The half life of C-14 is 5700 years and that of U-238 is 4.5 billion years. So U-238 can be used to determine the age of ancient objects. U-238 is used to date the age of the Earth. U-238 is found in volcanic rocks but not in fossils. Fossils contain C-14 only. C-14 is used to date fossils of recent age.