Because that's where the natural market state is. If developers could, they would save themselves the time and trouble of developing for multiple platforms, and they would choose the platform that let them extract the most value from their users. Everything else means less money for devs.
Just like Google created Android to protect its search business on mobile, they've created Chrome to provide a browser conducive to their business-- they are data merchants.
At the moment, developers aren't willing to sacrifice the lucrative market of iOS customers, so they ensure compatibility. It's one thing to tell customers to install a different browser to use their site, but it's another to tell them to use a different phone.
At the moment Safari marketshare is half on the desktop what it is on mobile.