CSI and Alias are equally amazing shows, and the only reason Alias has lower ratings is that, like others have said, wheras with CSI you can tag along from any episode, Alias is more complicated in that you can't really just drop in halfway through an episode and really know whats going on.
Actually, I tell a lie. Another reason why Alias has lower ratings is that some channels decide to give a quality programme a tottally erratic and crazy schedule, and then not even show the next season. F-I-V-E!!!!!
However if it's Alias and CSI:Miami you're comparing, Alias wins everyday!!!!!!!