I don't believe there is an official reason, other than it is easier to create a database column to store a binary object than to create a remote storage provider.
Also, historically when SharePoint Portal Server 2001 was released, SQL Server 2000 was the RDBMS platform and it didn't support EBS or RBS, functionality that was part of SQL Server 2005. None of the SharePoint products until MOSS had this support either,
and EBS with SQL Server 2005 was limited as well.
From an operations standpoint, storing the blobs in the database simplifies the solution. There are no "extra bits" you need to remember about, deploy, or support.
Jason Warren
Infrastructure Specialist
Habañero Consulting Group
www.habaneros.com/blog