I asked this question ( Why no Edge for Win 7) to one of the developers concerned with Edge development during BUILD Tour.

He told me that Edge is built on the Universal Windows Apps Platform which is going to be launched with Win 10. So because of its dependency, it won't be possible for edge to run on Win 7. The good thing about this is that MS is using the same platform that they will give to developers, to build Edge.

So people shouldn't hope that at some point Edge will come to Linux or Max OSX.