Old fashioned menu button
Back in 2012 google basically told developers that the Menu button was going away. Although it's not an official Android "rule" (not that I could find, at least), it's a strong Android guideline that phones no longer have menu buttons. It's a very recent development, and we should be seeing the majority of menu-button phones leaving the wild this year (my personal Galaxy Note 3 has a physical menu button but I'm due to upgrade in November). 

As for why, I honestly don't know. It's the subject of some debate, but nobody seems to be able to site an official Google source as to why they did it. Maybe it's so the bottom OS navigation bar on touch-screen only phones (like the Nexus 5 and 6, and the Moto X) could disappear without hiding app-relevant actions. Maybe Google wanted fewer buttons on phones with physical/soft button. Maybe Google just felt like it. I couldn't find any source that gave a specific reason as to why it was done.