This is a complex question that historians of slavery (and of the city) have been debating for quite some time, and it's not likely that we're ever going to reach a universally-accepted consensus on why Liverpool came to have such a near-total monopoly on the slave trade by the dawn of the 19th century. If you're interested in going into more detail on Liverpool's history and involvement with the trade, the best volume you're probably going to get on the matter is Liverpool and Transatlantic Slavery (edited by Anthony Tibbles, David Richardson and Suzanne Schwarz). This book was published in 2007 to coincide with the bicentenary of the slave trade's abolition by Britain and the United States. If you're ever in Liverpool, you also want to check out the International Slavery Museum, which is the world's only (major) museum dedicated entirely to the history of transatlantic slavery (though it also has galleries on modern unfree labour!). Parts of the collection deal with Liverpool's particular history and involvement in the trade, including one fascinating section on how many street names in the city honour people who invested in the slave trade.