The better question is why Nyjer Morgan doesn't steal bases every time he is on base. Nyjer could have stolen home plate easily if the Nats didn't decide that we wanted to show mercy on our guests from Arizona.
Unfortunately, Nyjer gets caught stealing way too often - picked off mostly. I'd prefer that he steal less, and pick the spots when he does more strategically. I like him running when the ball is in play, because he is so good at getting that extra base. Just his presence on the bases has an impact on the other team. His value is higher to me as a baserunner than a base stealer. The CS number takes away a lot of opportunities.
To Morgan's credit, the most promising thing about him offensively has been his OBP since joining the team. His plate discipline and willingness to bunt for hits has really helped. As a light hitter, he needed to get that OBP up to make him better than a marginal leadoff hitter. He's been a good tablesetter so far in this lineup. I doubt he will sustain the hitting pace, but if he can keep getting on base as a consistently, he'll be a bargain.