Why am I unable to roll back Lumia 520 from Windows 10 tp build 10166 to Windows 8.1.?

[shoaib9]

Unable to roll back Lumia 520 from Windows 10 tp build 10166 to Windows 8.1.

I installed Windows 10 on my Lumia 520 but its lagging too much. So, I wanted to roll back to 8.1 using 'Windows Phone Recovery Tool'. But after downloading the 1.5GB installation file it failed to install it on my phone and started to download the file again.