First of all:Why? and Why should they? Bismark had a very short carreer at the very beginning of WWII. Iowa served off and on up until the 90s. The Iowa class BBs surely have much more signifigance in US history than the Bismark did. They've also appeared in modern cultural settings more often than Bismark.
US History is significant, I agree, to the whole world. It´s undeniable. But is not the ONLY history that matters. The position that the US has nowaday deserve more to the mistakes of others rather to an incredible strategic and cunning behaivor by the US. Mistakes that the US is trying to emulate right now, as a matter of fact, carving it´s path to decline.
Bismarck was a battleship that fought as one, as it was designed for. Bismarck perpetuated her name in History (not US History alone) for what she achieved at Denmarck Straits battle and the hunt the British gave to her. Bismarck success and demise was as a fighting battleship. She fired her guns against her equals and was fired upon by her equals... and a lot other vessels and devices. Naval History books will always have a chapter for Rheinubung and their participants.
Iowas never fired their guns at any other battleship, less to sink one of their... equal foes. Of course they are important to the US history. But Bismarck is more important to Naval History than all the Iowa sisters put together, because in WWII they did nothing worth than to be incredible big yachts for admirals and Douglas McArthur. Of course they were good gunnery plataforms for troops landing on Pacific Islands in WWII and for launching long range cruise missiles at Iraq. But they will never were baptised in the fire of real combat, being themselves at danger of being sunk by her contemporary adversaries, even having a chance or two of doing so.
So, Bismarck is more important than ANY Iowa in Naval History. And Yamato was more powerfull a Battleship than Iowa in Naval History.
So it was written... so it was done... with History as my witness.