Lamb and the USA

On a recent trip to the USA I was struck by the absence of lamb on restaurant menus. This PDF on meat consumption trends underscores my point. The amount of lamb eaten in the USA is vanishingly small. Given the mixed nationalities who migrated west in the 1800 and 1900s this seems curious. Lamb is still popular in the UK, even if we traditionally have it with mint sauce. The French and Italians both treat lamb well and the eastern Mediterranean countries all cook lamb regularly. It is even a staple in Irish stew. All of these nationalities contributed to the country that is now the USA, yet lamb has gone missing.
After a quick serendipitous chat with Thomas Vander Wal, it seems lamb is commonly available in upstate New York and in California, but beef is the main elsewhere. So what happened that beef, chicken and even pork and turkey dominate so much in the USA. Lamb is a lovely meat.