I used to have iPods, and I just got tired of the Apple's proprietary ecosystem. My first smartphone was a Palm Centro that was fantastic for the time. It was not as smooth or pretty or easy to use as an iPhone, but it was far more capable.
My next two phones were Blackberries provided by my employer. In stock form they weren't terrible, but securely locked down they were awful. I feel Blackberry's security played as big of a factor in their demise as their rise. A locked down Blackberry was such a poor device that when BYOD came around people couldn't get rid of them fast enough.
Next I went with an EVO3D. I chose it over a WP phone because WP didn't have the email client I required for work at the time. It was an OK phone but suffered all the classic Android issues of the time with a poor camera, lag all over the place, and poor support. (although I don't care what anybody says, that 3D was pretty damn cool for family pictures and videos)
For my last phone I looked VERY hard at a Nokia Icon because I love Windows 8.1 on my Surface and find Windows Phone to be just as fast, fluid, and easy. But WP just wasn't quite there yet and I wanted Chromecast, Swipe, and water resistance. My wife ended up getting an Icon and it's a very nice phone but WP still has too many shortcomings for me. VERY close, but at this point I honestly don't think WP will ever close the gap. The tiny iPhone 5S was never a serious consideration. Way too small.
So I have my S5. It's not perfect by any means and it really irritates me that a phone with such great hardware still has lag and responsiveness issues. But it does everything I need it to and then some and is generally fast and easy to get it to do what I need it to.
Apple does keep improving but it seems like by the time they offer what I want there are other things I want that they don't offer. From Verizon/Sprint availability, Copy and Paste, Office, larger screens, 4G, untethered setup, Swype, NFC, and a host of other little things, they have always been a step behind. Combined with their walled garden, they have just never really appealed to me. I will admit they are quite nice at what they do but I refuse to support such proprietary ecosystems anymore. That's bad for consumers in the long run.