FM radio requires a large antenna. It will not work without one. The 5th generation iPod nano (2009) was the first iPod with an (enabled) FM radio. We used the headphone wire as the antenna. It worked well enough. While there was a tiny speaker in those iPods, headphones were the only practical way to listen to audio.
That's not the case with phones. People listen to the built-in speaker, and they listen over bluetooth and WiFi. It's becoming less and less common to plug headphones into these devices. That being the case, including an FM radio feature would just be a horrible experience for the majority of users: they'd hear nothing but static, all the time. I don't know about the other smartphone makers, but Apple will not ship something that will likely be a horrible experience for most users. Couple that with very low demand from users (as the article points out) and there's no FM radio on iPhones.