Sure, iCloud kept files in sync (usually), but the iPhone was the juggernaut. It was because it hit the perfect sweet spot of company, market, and value chain.
Company: Apple from the very beginning has been premised on the idea of integrating hardware and software, and the iPhone was the ultimate expression of that premise.
Market: The smartphone market was the best market technology has ever seen: not only did everyone need a phone, but in developed countries carriers subsidized top-end models because they drove higher average revenue per subscriber. Moreover, because a phone was something you took with you everywhere, there was far more value placed on non-technical attributes like fit-and-finish and brand.
Value Chain: Apple’s integration delivered sustainable differentiation in the smartphone value chain, forcing every other element, from suppliers to network providers to app makers to modularize themselves around Apple’s integration.
The result was the most successful product ever.