While Linux includes backup software, many people choose a commercial package because they find it easier to use. There are lots of alternatives including Windows2000 and Solaris-based systems. While Linux isn't the only answer, it is an excellent answer. Linux has proven reliability offering typical uptimes in the hundreds of days. The cost of a Linux solution is lower than that of the alternatives.
Cost deserves more discussion.
Besides a zero cost for the software, Linux tends to be frugal in resource utilization. You can squeeze more performance out of your hardware with Linux than say Windows2000. As an example, I know of two ISPs in Vancouver. They have about the same amount of traffic to handle. One ISP uses nine WindowsNT systems, the other uses a single Linux system.