The 2 main reasons are
- Information Schema views are defined by the relevant ANSI standard and are available with any RDBMS that supports those standards
- Microsoft have done the hard work to sort out the details that are exposed in these views (if you use system tables, you need to do this.
Another reason that in my opinion ranks well behind the above two is that, supposedly, Microsoft do not support the use of system tables and the content of the system tables may change. Practically, I am not convinced that either of these is really true. In SQL 2005, system tables have been more formally made available to you and both of these seem to be non-issues.