Primarily because German panzer assets were centered on Heeresgruppe Mitte in 1941-42 against Moscow, and then with Heeresgruppe Süd in 1942-43 for the offensive against the Crimea, Rostov, the Caucuses oil fields, and Stalingrad. The capture of Leningrad was never seen as a game changer either. Moscow had once again become the Soviet capital in 1918, lend lease came through other routes, and the Soviet war economy was located by Moscow, the Urals, or the Caucuses.

The Siege itself was initially an ad hoc development, as panzer units were successively drawn away from the North to aid in operations deemed more pivotal, such as Kiev, Rostov, and Moscow. With fewer panzer divisions available to create operational flexibility, Heeresgruppe Nord lost its ability to flank, and it could only smash apart Soviet defensive lines with unacceptable infantry losses. The campaign settled into trench warfare attrition.

Eventually the Germans did adopt a plan, Operation Northern Lights, to directly attack Leningrad with panzer and artillery reinforcements from Manstein's 11th Army in the late summer of 1942. However, this operation was delayed and then nixed, first by a Soviet counterattack from Leningrad itself, then by successive Soviet counterattacks in Operation Mars and Uranus that winter which once again drew reinforcements from Nord to the other two fronts. After this point, Soviet quantitative and qualitative superiority prevented offensives.

This is not to say that alternatives weren't available. Both the German war economy and military failed to properly mobilize all conscript and material assets until disaster threatened in 1942-43. Large numbers of panzer units were wasted in garrison duty throughout Occupied Europe. But under the constraints of their resources on the Eastern Front, the German Heer had to shuffle forces about, and Heeresgruppe Nord was always in last place.