Honestly windows phone was sabotaged by both 3rd party developers who refused to port their apps to the platform, despite how easy it became AND did things like kill their own APIs to stop other developers from developing ports themselves, as well as by Google by not allowing their services on the platform.
The hardware was honestly top notch, even compared to my current S21 ultra. They were fairly pro consumer, having removable batteries, SD card slots and a 3.5mm jack right up until the end, even after the other big manufacturers removed them. And after the major update (the WP 8.1 update iirc) the software was really nice, intutive and pretty. I miss arranging and resising tiles, I miss having my pictures or album artworks showing up on the homescreen and I still use the Microsoft launcher on android to get the app drawer like I had on my WP.
Windows Phone was mostly sabotaged by first-party developers. Microsoft has a history of abandoning their mobile phone OSes after very short periods of time and nobody trusted them not to do it again. As a result few app developers bought into the ecosystem and smartphone enthusiasts told their friends not to get Windows phones, causing modest sales, causing Microsoft to immediately drop the platform.
As everyone expected them to.
