Why does it take longer for drugs to be absorbed orally?

First, if it's a pill, it has to dissolve in the body. Absorption through the stomach is slowed down by the mucus lining of the stomach and whatever else is already in there.

A lot of drug absorption takes place in the small intestine. Some pills are coated with a substance that stops them dissolving until they reach the small intestine. But they won't get there until the stomach pushes them into it, and the stomach leaves whatever arrives in it lying around for a while.