There are a myriad of reasons why, but I'll try and list a few to get you started on some further research (armchair historian here):

• Germany wasn't functioning on a 'Total War' economy until quite late into the war, hence some lack of raw materials owing to having lost so much in Russia and being under so much pressure from the RAF and USAF. This would be exacerbated by a couple of other factors: Plain arrogance - the "Lunch in Paris, Dinner in St. Petersburg" mentality of Kaiser Wilhelm recast, i.e. the High Command's faith in their own generalship and the quality of their economy led them to believe the war would be won with what Germany already had - and Hitler's overbearing megalomania when he declared war on America, which bolstered naval protection of Atlantic shipping and sealed the fates of over thirty thousand German sailors.

• The Royal Navy was, at least in the pre-war era, THE naval power on earth and was the envy of the Kriegsmarine. Happily for the Nazis, the Treaty of Versailles allowed the Kriegsmarine to build a submarine fleet the same size of the Royal Navy. However, the Nazis had a bad case of Victimhood Complex and were eager, at first, to not overreach on what they had built. The reason for this was that there were many forces in Britain and elsewhere that were busily committed to rearmament and were looking for a highly militarist Nazi Germany to make a fatal miscalculation. Numerous accounts from the German Leadership, from Hitler on down, have stressed that had he been resisted earlier, and more determinedly, he would have had to have backed off. Indeed, at the time of the Munich Conferences, a cabal of German officers contacted Neville Chamberlain to say that they could overthrow Hitler in a coup, should British forces be committed to the fight for Czechoslovakia.

• Before - and slightly after - the declaration of war on America, allied shipping was incredibly vulnerable to U-Boat attack and there seemed no point in applying further pressure on shipping.

In brief: An economy that wasn't geared for a large war effort, a war effort that was too great, a set number of U-Boats that did not increase for fear of international reprisal at first and for arrogance later.

It would have blocked off supplies to Great Britain.

To be clear with the existing industrial base, size of their naval establishment to crew and train an effective force, and technology available this is a highly dubious assessment.

In reality in the 26 full months between the invasion of Poland and the entry of the US into the war, at which point the survival of Britain was essentially foregone, the Kriegsmarine sunk enough shipping to outpace production just 7 times. Not enough to force starvation, or ruin rationing schemes, or prevent the functioning of the Imperial war effort, just to slowly reduce the amount of shipping afloat for use by the UK.

If we assume then that in the run up to war all the major surface units were canned in favor of subs then they could have possibly doubled the number of boats ready for war, or even we can be generous a little more. Around 75 were in service in life, so even with 200 boats ready for war, its a very one dimensional threat and hard to imagine them all being ready or of the quality that a small selective service branch can foster.