The answer is as others have stated appl not supporting the open standard RCS.
I will elaborate with apple are deliberately dragging their feet supporting standards as a deliberate attempt to put social pressure on you to buy an iphone.
an audience member asked Apple CEO Tim Cook for some tech support. “I can’t send my mom certain videos,” he said; she used an Android device, which means she can’t access Apple’s iMessage. Cook’s now-infamous response: “Buy your mom an iPhone.”