I asked myself this question recently as well. At first, I looked at why row-based replication wasn't enabled by default. Obviously, row-based replication creates the possibility of IO-strain on disks by the amount of data that potentially could be written to the binary log.
Not surprisingly, this is the number one disadvantage of row-based replication the MySQL documentation mentions. A bit further down, another disadvantage would be a potential issue with performance with MyISAM:
For tables using the MyISAM storage engine, a stronger lock is required on the slave for INSERT statements when applying them as row-based events to the binary log than when applying them as statements. This means that concurrent inserts on MyISAM tables are not supported when using row-based replication.
MyISAM is the default engine of the 5.1 series. It is not until 5.5 that InnoDB is default. This replication impact on MyISAM, to me, is a strong reason not to use RBR by default.
But your question revolves around the MIXED format. Under MIXED format, the default is to use statement-based replication until a statement is called that required row-based replication. The link above gives a nice overview of what type of logging gets performed on certain situations (safe, unsafe, row injection).
I think there are too many potential situations that would cause replication to switch to row-based replication, causing some of the performance issues noted above on non-optimal hardware. And let's face it, MySQL has a large following of users running it on non-optimal hardware.
This situation stands out to me the most:
If a statement is logged by row and the session that executed the statement has any temporary tables, logging by row is used for all subsequent statements (except for those accessing temporary tables) until all temporary tables in use by that session are dropped.
Note that all statements involving temporary table in row-based logging is considered unsafe, so this could cause issues with your replication setup.
Ultimately, I think it is a good idea to have STATEMENT be the default, having to make a conscious, educated decision to switch to MIXED.