I think it was for the same reason why Michael himself had to whack Sollozzo and McCluskey in Part I. Rocco was a caporegime in the family, and everyone KNEW he could get the job done. They couldn't take the chance that a rookie, even a very good one, might miss or not kill Roth. So it had to be Rocco, even though he probably knew his chances of coming out alive were slim to none.