As the old saying goes, "Necessity is the mother of invention."
At the time the Germans did not need all of the gadgets that were packed into the Type XXIs.
Schnorkels are nice to have when you cannot spend to much time on the surface for fear of an undetected attack(either surface or air), and this did not really occur until late 1942-1943. The American submarines never made use of "snorts" in the Pacific, but then again the Americans had better radar and radar detectors on their submarines(not that they were "proof" against an unexpected attack, but they helped a good deal). They Germans OTOH increasingly faced mounting British air and surface patrols, and by 1943, remaining on the surface for any extended period of time invited a British attack. Hence, the need for a schnorkel wasn't there in 1940, but in 1943 it was almost a necessity to survive.
The same goes for the high underwater speed. In the beginning of the war, a high surface speed was necessary for hunting the British shipping. However, by 1943, high underwater surface speed was necessary to evade the ever present British ASW assets conducting depth charge attacks, especially with the advent of the hunter-killer groups.
Finally, the Germans had their own problems with the Type XXI, which is why only two(U-2511 & U-3008) were on patrol when the war ended(even though they were being commissioned as early as June 1944).