Seagate stopped making 7200 rpm 2.5" HDDs in 2013 [1]. I assume Microsoft didn't want to pay more for a 7200 rpm 2.5" WD Black HDD so that leaves 5400 rpm drives as the only option.
The hard drive in the Xbox One is a 2.5" 9.5mm Samsung Spinpoint M8 drive [2]. Seagate purchased Samsung's HDD business in 2011 [3].