Agree with tinstaafl, although the reasons vary between deltas and canards as he says.
With a delta, frankly it's not needed. They keep lifting to higher and higher alpha, eventually stalling around 30-35°. There's no need for flaps, the main problem is still seeing the runway when you're that nose-up (hence the famous Concorde drooping nose).
With multiple wings you have to match the lift of each or you get a huge, and potentially uncontrollable pitch change with flap selection. This is very hard to mechanise successfully, and I'm not sure that anybody's ever really succeeded.
I believe Darrrol Stinton's done quite a lot of work on the latter subject, although I'm not sure where he published it.
G