Why do I keep using it? Familiarity and I like the simplicity of the OS. It does have a few essential apps like offline maps. Windows Phone OS is probably the best UI that Microsoft created. It's also refreshingly different compared to the converging design and behaviour of Android and iOS (grid of icons vs tiles).
Ironically, a little-used, dead-end OS with few apps available on the platform means you can't do a great deal on the phone apart from call or text people. Maybe a blessing in disguise?