Oh the reasons are legion, and I'll begin by asking a similar question: Why didn't the battle of France begin right after the Invasion of Poland? Almost a year passed then, too.

Well, the simple answer for both cases is the Wehrmacht was operating at a tempo in 1939-1940 that wasn't sustainable for anything beyond short bursts. There's more to it than that, however. Just like in the pause between Poland and France, the pause between France and Barbarossa was a time of absorbing and training replacements, upgrading aging equipment (and bringing up equipment still in use to acceptable standards - mobile units took a high amount of wear and tear in France) and absorbing lessons.

There's also very unique considerations for Barbarossa. This was one of the largest land invasions in human history, one shouldn't go into it lightly or hurriedly; and the Germans (and it must be said, the world) figured they had time on their hands. The RKKA's abysmal state was well known, and it was presumed that their massive deficiencies wouldn't be rectified in short order. The Germans were ultimately proved quite wrong about the sorry state of the RKKA, but at any rate, the Soviets were not expecting a German attack so soon and were incapable of effective offensive operations themselves. The Germans therefore had time to prepare, plan and build-up.

They needed it. The Invasion of Barbarossa would involve a multi-national effort, and would eventually commit in excess of 4 million Soldiers running from a front stretching from the Baltic to the Black Sea. The scope and scale of the invasion meant that a period of stockpiling, equipping and training was absolutely tantamount: If anything we should be expressing shock that the Germans only spent a year preparing for such an endeavor.

The Germans weren't completely idle in this space of time either; the intervention in Greece, Yugoslavia and North Africa are part of a larger build-up for Operation Barbarossa, but its doubtful that it had any real impact of the planned timetable of the invasion. Barbarossa was launched essentially on time and in the most favorable of conditions.