Most Apple followers will tell you this is because of battery life concerns.
However, sleep tracking based on movement is not a precise science, and can even make some
people's sleep worse (recommend you read the full article and follow the links to the various studies). Real sleep tracking requires monitoring of brain signals.
So based on how Apple have approached other forms of health tracking, I like to think they'll hold off until they have a product that is actually useful and likely to have a positive impact on people.
It boils down to, with the exception of a few rare conditions, if you wake up feeling refreshed you had a good night's sleep, if you don't feel refreshed you probably had a poor nights sleep. You don't need a fitness tracker for that.
And yes, I understand that Apple own the sleep tracking company Pillow, but I reckon it's kept at arm's length for a reason.