Why isn't partition-level lock escalation the default?

Question

In SQL Server, locks are normally escalated from row or page -> table. Starting with SQL Server 2008, a new level of lock escalation was added - partition level.

However, this isn't automatically enabled for partitioned tables - by default, the table is set to skip partition locking and go right from row or page -> table. Why would this be the case? Is there a reason why I wouldn't want to switch all my tables from TABLE to AUTO so that they escalate locks to partition level instead of to table level?

Since the default is still TABLE, I figure there must be something I'm missing about the downside of AUTO.