Most advocates of the 3-4 defense seem to feel that high draft picks are wasted on D-linemen. Most D-linemen taken early in the draft are picked because of their ability to rush the QB and rely more on speed, and guys more suited for the 3-4 are usually available in later rounds. Also, 3-4 advocates seem to prefer veterans to rookies because of the strength requirements of the position.
The higher picks on a 3-4 defense tend to be corners (especially first round picks) and then linebackers. Safeties and D-linemen are picked later in the draft.