Mostly because they've been reliable. They contain Toshiba hard drives, since Toshiba is a drive manufacturer, unlike some third party portable drive makers where the drive in the box might be from different companies. They're available in larger sizes (for a 2.5" drive), up to 1 TB. They're often on special at Newegg, so can be fairly inexpensive. Unlike some other brands (Western Digital, for one) they don't come with any difficult to remove 'virtual CD' partitions containing application software.
Also, they work with the Squeezebox Touch, whereas a couple of other brands of bus powered drives did not. Probably just means that they require less power to spin up.