Orcs have lousy memories.
No. That's not right.
Orcs have superb memories that retain far more detail than that of the average human. If you ask an orc about his breakfast, he's likely to be able to tell you how many bites it took him to eat it.
That's assuming he didn't take a nap in the meantime.
For orcs, sleep works as a memory diffuser. It works gradually. An orc will remember yesterday about as well as a human. A couple of days before that will be fuzzy. Anything before the last week is a blur.
Unfortunately for orcs, this has made them prized as slaves - for both labor and warfare. After the first week of slavery, they barely remember another life.
Reminders help, though. Stories. Visual cues. Notes. They'll dig up old memories (or at least memories of old memories) that the orc can't normally access. This is why so many orcs get tattooed. If there is something that they want to remember, they mark themselves with it. When they see the tattoo, the memory comes back to them.
This is also why orcs, at least the literate ones, keep diaries.