Rackmount servers and the 19-inch rack standardized several key dimensions on the server form factor - the width and the height (1.75 inches per "U"). Driven by the popularity of Cobalt's RaQ servers (now departed), many of those 19-inch racks filled with 1U "pizza box" servers, which offered a way to cram a lot of computing power into a rack. Or so we thought in those crazy pre-blade days.
But filling a rack with 1U servers is more complicated nowadays, as Chuck Goolsbee notes in a feature over at the Server Specs blog:
My biggest beef here is the 1U servers that seem to be growing to absurd depths. The worst offenders I'm dealing with at the moment are Dell's 1950 and Apple's latest version of the Xserve. Both arrive at 30? (76.2cm) or longer. I'm sure there are others that have reached these lengths too. They have roughly the same form factor as the flight deck of an aircraft carrier. 1U x ~18? x REEEEAAAAALLLLLLLYYYYY LOOOOOOONG. ... When 1U servers started appearing they were rather compact, akin in form factor to your average Ethernet switch. 1U x ~18? x ~12?
That's one reason why we're starting to see deeper racks and cabinets. "With deeper servers and larger volumes of cable, companies are continuously looking for ways to maximize space in their enclosures," noted Pat Johnson, vice president, Rack and Cooling Solutions, APC-MGE, who noted that APC's NetShelter SX 1200 mm Deep Enclosure is 130mm deeper than the standard 1070mm (42-inch) enclosure.