I guess right now the big main reason is support. It a short period of time Oracle will no longer Support 8i. What this means is that there will no longer be patches created for the database. So if in the future you run into a Bug in 8i that makes it impossible to do your work, you will have to upgrade because oracle will not fix it. You will still be able to use Metalink for other support, until a time when they stop that too.
In 9i there are some great enhancements to SQL like the MERGE statement, actual Ansi Joins using "INNER JOIN", "FULL OUTER JOIN" in the FROM clause. If you want to do a full outer join in 8i, you will need a left outer join on both sides and use a UNION. The CASE statement is also a well needed enhancement.
Those are just two. Support being the most important. Unfortunately when you buy Oracle you have to keep giving them money.
Mark