Jubal Early's plan was multifaceted and among other things, he didn't want to attack in the obvious places--from the south of west. After all, who would expect the Southern army to attack from the north? Of the weakly manned DC defenses, those to the north were more vulnerable than others. Early's reconnaissance proved this (at least the day before the battle!) Also, Early hoped to meet a naval force at Point Lookout, Maryland, where he could liberate and arm thousands of Confederate troops. The path he took would have put him in position to do this. It's also important to note the Early wasn't "targeting" Fort Stevens in advance. In fact, on the morning on July 11, 1864, Early had scouts probing all along Washington's northernmost defenses, from Fort Reno (which guarded the important roads to Georgetown) out to Fort Lincoln. In the great "Mr. Lincoln's Forts," the authors write that Fort Reno was originally scouted as the site of Early's main attack, but its stout defenses (including a number of howitzers) made Early's scouts head further east, towards Fort Stevens.