Linux?

From IBM we get:

"Inside the beast itself, Sequoia is expected to contain more than 1.6 million processors, thought to be IBM Power chips. The system is also reportedly to have 1.6TB of memory and will run Linux. The open source operating system commands approximately 85 percent market share in the high performance computing field. Windows meanwhile is relegated to less than five percent market share, according to industry estimates."


Why Linux?