The huge Windows Phone 8.1 release made Windows Phone a very different platform than it had been before. As well as high-profile new features, most notably the Cortana digital assistant and major work enabling developers to use substantially the same code on both Windows Phone and Windows 8, it had one more important feature: it was cheaper than ever.
2.4 GHz 802.11b/g/n, Bluetooth 4.0 LE, GPS, GLONASS, BeiDou
3G (up to 21 Mbps HSDPA)
3G (up to 42 Mbps HSDPA)
4G (up to 100 Mbps LTE)
Micro USB, headphones, microSD
5MP rear camera, f/2.4 aperture, 720p video
It's not simply that the software itself costs nothing, though that certainly helps. It's that Windows Phone 8.1 changes the hardware spec in some important ways.
Windows Phone launched needing an array of Windows Phone-specific buttons: the trio of buttons on the front, for back, Windows, and search, and the camera button—with half press to focus—on the side. Though the camera button requirement was relaxed some time ago, Windows Phone 8.1 went a lot further: the only mandatory buttons are the volume rocker and power button. The camera button and front buttons can be discarded entirely, with the former replaced by a tile for the camera app, and the latter by three on-screen buttons.