Why are cells arrested with colchicine during metaphase of mitosis when creating a karyotype?

Because in metaphase the chromosomes are highly condensed and easy to see down a microscope - so a karyotype can be easily produced. The addition of colchicine to a culture of cells means that all of the cells will be arrested in metaphase, so when you look down the microscope you have a large number of cells at a stage where there chromosomes are highly visible.