Windows 7 and 8.1 users are not allowed to download the Windows 10 Edge Insider preview for Windows 10 users. Microsoft is blocking Windows 7 users from downloading the browser from their web site. It is not known why Microsoft is preventing older Windows versions from getting the Insider build, but it is probably because there are some features that do not work properly on the older operating systems. For those who still want to test it out, you can download the Developer or Canary builds and give them a try.
Source: https://www.bleepingcomputer.com/news/microsoft/windows-10-microsoft-edge-installer-also-works-on-windows-7/