Why the slow rate of adoption? Analysts point to a few possible reasons. One, the recently released iOS 8 requires more memory.
"You need a lot more memory to download this version, and you might need to move a lot to the cloud to make room for it," says Tim Bajarin of Creative Strategies. "Some people might not know how to do that."
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iOS 8 requires 4.6 GB of free space before users can install the operating system, and many iPhone users don't have that amount of space remaining on their devices. iOS 7 required 3.3 GB of space.
Second, analysts say initial snafus with the rollout of the new operating system might have alarmed some users.
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iOS 8's launch on September 17 including a bug that forced Apple to pull HealthKit-enabled apps from the App store.
After that snafu, Apple pushed out a patch on Sept. 24, but it too was quickly pulled after iPhone 6 and iPhone 6 Plus owners voiced complaints of non-functioning cellular and TouchID hardware.
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The company issued the latest iOS8.0.2 patch on Sept. 25, with fixes for the HealthKit, cellular and TouchID problems.
"They did have failed updates," says Horace Dediu of Asymco, a mobile software development company. "So people could be more cautious."