Google Inc. (Nasdaq: GOOG)’s Android operating system passed Apple Inc. (Nasdaq: AAPL)’s iOS as the most popular software platform for application downloads as consumers bought more Android smartphones.

The Google platform accounted for 44 percent of all app downloads in the second quarter, eclipsing Apple’s 31 percent share, according to ABI Research. That was fueled by a 36 percent jump in Android phone shipments from the previous three months, ABI said.

Why?

“Android’s open source strategy is the main factor for its success,” said Lim Shiyang, research associate. “Being a free platform has expanded the Android device install base, which in turn has driven growth in the number of third party multi-platform and mobile operator app stores. These conditions alone explain why Android is the new leader in the mobile application market.”

Another factor is smartphone sales.

“iPhone shipment growth in Q2 2011 slowed to 9% from 15% a quarter earlier,” ABI reported. “In contrast, Android smartphone shipments increased 36% in Q2 2011, compared to 20% in Q1. Android’s install base now exceeds iOS by a factor of 2.4-to-1 worldwide; by 2016 this factor will grow to 3-to-1.”

Google, based in Mountain View, Calif, introduced Android in 2007 as an open-source platform, allowing any phone maker to use it for free and focus on designing hardware rather than software. Android has helped propel Samsung Electronics Co. into the ranks of top smartphone makers and drive a recovery at Motorola Mobility Holdings Inc. even as sales of Apple’s iPhone surged.

“Android’s app downloads per user still lag behind Apple’s by 2 to 1,” said Dan Shey, practice director of mobile services at ABI.

Global downloads of games, music, news and other apps are expected to rise to 29 billion this year, more than triple the 9 billion downloaded in 2010, ABI said.

(Read more from ABI here.)

(Bloomberg news contributed to this report.)

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