In the blog post about the release of Windows 10 Preview Build 9879, Microsoft announced that they will be releasing ISOs for the Windows 10 Preview Builds in the future.
And finally – we heard from you loud and clear you want ISO images of the new builds we release. We felt it was important to listen to that and give you what you want – but there’s a catch. Getting the update & install data from our Preview Builds mechanism is super important for us. It helps us ensure smooth ESD distribution, download, and upgrade success for this program going forward, and also will help us ensure great upgrades for people once we release Windows 10. So we’re going to release the ISOs at the same time as we publish to the Slow ring. That means if you want to be FIRST and FASTEST to get the build, you’ll need to use our Preview Builds mechanisms (either automatic or Check Now in PC Settings to download.)
Microsoft also confirmed that it will be the last new build of the year for Windows Insiders and they will have something new to share with us early in 2015. During the time from now to 2015, Windows engineering team will be developing and refining the product further.