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

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. 
I hope I helped you :)