The Bismarck was a great threat to British shipping because, as a battleship, it had big guns which could fire high explosive shells at targets 10 miles or so away. The British cruisers which came after it were armed with smaller guns with significantly less range (I don't know what the size and range difference was). The cruisers, in order to fire on the Bismarck, had to get well within the range of the big guns (easier then than now: darkness offered protection, as radar had not yet been developed). For the cruisers to target the Bismarck made them targets of the Bismarck. The cruisers had to get off enough effective fire to sink the battleship before it could get its guns trained on them.