In a word, sales growth. (actually that's two words but who's counting.)
[link|http://news.com.com/2100-1001-823787.html|Linux on the Mainframe enters phase II]
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"Linux has been a big breath of fresh air for the mainframe," said Giga Information Group mainframe analyst David Mastrobattista. On a scale of one to 10, software companies' enthusiasm for mainframes had for years been down at one or two. "Now that Linux has arrived, I'd put the enthusiasm factor around seven or eight. It's definitely moved into high gear."

Gartner Group estimates that 70 companies employ Linux on mainframes for real-world use, with several hundred more evaluating the operating system. Financial services giant and Citigroup subsidiary Salomon Smith Barney is among those using Linux on IBM mainframes, an IBM representative said.
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Though some technophiles use Linux on desktop computers, it's best suited for use on higher-end computers such as workstations for programming or mechanical engineering or networked servers that handle networked chores such as sending e-mail, recording sales orders or communicating with automated teller machines.

IBM leading the charge
IBM, largely the last company still selling the refrigerator-sized mainframe servers, is leading the Linux charge with its wholehearted adoption of the OS and a new Linux-only mainframe. Big Blue's Linux mainframe project began by making sure Linux would run on the zSeries line. Then, part of the $1 billion that IBM spent on Linux in 2001 went toward moving over higher-level software such as the DB2 database package and the WebSphere e-business software.

IBM has a powerful incentive for the movement: Rejuvenated software for its z900 mainframes means big money for Big Blue.

Bill Zeitler, head of IBM's four server groups, credits Linux and WebSphere for driving much of the 13 percent growth in IBM's mainframe revenue sales in 2001. "This is important not just because it was the only (server segment) that grew; it's the first time in 13 years that mainframe revenues grew at all," he said.