What will cause no lock escalation to table lock in deleting more than 300,000 rows in a table in SQL Server 2008 R2?

I deleted 300,000 rows in a table in SQL Server 2008 R2.   I checked the locks information by querying sys.dm_tran_locks when the delete statement was running.

delete top (300000)  from log where logdatetime < getdate(); -- there is a non-unique, non-clustered index for column logdatetime

I expected to see object/table exclusive lock but only found page and extent exclusive locks.  I have checked that the lock escalation of the table log is still set to default (table).   What are the possible reasons for no lock escalation to table lock?