Irish Passenger Lists, 1847-1871

Although ACOM gives no decription of the dataset, it is evidently taken from the lists that were developed from the order books of two main passenger lines operating out of Londonderry--J.& J. Cooke (1847-67) and William McCorkell & Co. (1863-71).

Why, therefore, are records of passengers sailing from Londonderry to America any less of a US collection than the NARA passenger lists?