The decline of OS X Snow Leopard has accelerated in the last three months, perhaps because users have realized that Apple has stopped patching the 2009 operating system for the Mac.

From March to May, Snow Leopard lost 3.8 percentage points of its share of all Mac operating systems, according to data from metrics company Net Applications. That's the largest three-month decline for Snow Leopard since the stretch of August-October 2012, shortly after the introduction of OS X Mountain Lion, the July 2012 upgrade that was priced 33% less than the previous year's Lion.

Net Applications estimated that 15% of all Macs that went online in May ran Snow Leopard, aka OS X 10.6.

Snow Leopard has become noted for its stubborn resistance to retirement. Even though newer editions have shed share faster -- both Lion and Mountain Lion accounted for smaller percentages of all Macs last month -- Snow Leopard has refused to go quietly into the night.

Users have given many reasons for hanging on: The OS suited their needs; their Macs, while old, showed no sign of quitting; or they disliked the path that Apple's taken with OS X's user interface (UI).

Snow Leopard was also the last edition able to run applications designed for the PowerPC processor, the Apple/IBM/Motorola-crafted CPU used by Apple before it switched to Intel in 2006. Additionally, Snow Leopard was the last able to run on Macs equipped with 32-bit Intel processors.

But one relatively recent reason to dump Snow Leopard may have outweighed those factors: Apple has stopped crafting security updates for the five-year-old OS.

Apple shipped the final security update for Snow Leopard in September 2013. Since then, it's declined to patch Safari 5, the browser bundled with the OS, four different times, and ignored the operating system twice when it's patched the newer Lion and Mountain Lion.

While news about Apple cutting off Snow Leopard from security patches has attracted less attention than, for example, the retirement of Windows XP -- largely because Apple, unlike Microsoft, does not spell out its support policies, much less publicize an end of support -- that news has slowly spread.

Another possible reason for Snow Leopard's faster decline of late may be economical: In the last eight months Apple has cut prices of its notebooks. In October 2013, the Cupertino, Calif. firm dropped prices of its Retina MacBook Pro laptops by up to 13%. And two months ago, Apple reduced MacBook Air prices and offered a sub-$900 model to the public for the first time. The price cuts may have prompted Snow Leopard laggards to replace their aging Macs in larger numbers than previously.

At its current pace, Snow Leopard will fall under the 10% mark by November, and below 5% by March 2015. But users may pick up that pace even more if Apple has its way. OS X Yosemite, the upgrade scheduled to ship this fall, includes some flashy new features, "Handoff" in particular, that require a newer Mac. Only machines that were launched two or more years after Snow Leopard will support the start-a-task-on-one-device-resume-on-another Handoff.

OS X Snow Leopard's decline in Mac user share has accelerated over the last three months. (Data: Net Applications.)

Gregg Keizer covers Microsoft, security issues, Apple, Web browsers and general technology breaking news for Computerworld. Follow Gregg on Twitter at @gkeizer, on Google+ or subscribe to Gregg's RSS feed. His email address is [email protected].

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