Can't install Windows 7 on Macbook Pro
Steve Chambers is right. 

As Windows release new versions boot camp is updated to support them and more often than not they drop support for the previous version of Windows. 

You could either install Windows 8.1 (it still has the start menu ;) ) or install Windows 7 under parallels desktop. 

I personally use parallels desktop as it allows me to use Windows and Mac at the same time. I run it on my 2014 MacBook Pro retina. No issues, everything runs perfectly.