Because in this way, MS will build very good version of product without carrying any historical "sins" along. By taking this as the advantage they will only concentrate on the SharePoint 2010 support and will make really a very good product. So, how to get the support for SharePoint 2007 sites? Is there no other way to edit the earlier version sites? I know this is the question in your mind now.
Don't fear. There is an option. By thinking these questions in mind, the SharePoint designer team is developing the software/product to install the designer side by side. So, you can install the SharePoint designer side by side the both versions on the same machine without having any issues. So, this will solve all the problems. If any site which is of earlier than SharePoint 2010 you can use SharePoint Designer 2007, for SharePoint 2010 you can use SharePoint designer 2010.
Are there any requirements for side by side installation? Yes, it has some requirements. You know a big change from previous version to current version is bit [32 bit vs 64 bit]. As SharePoint Designer 2007 is came only on 32-bit[x86], you will only install SharePoint Designer 2010[X86] 32 bit version side by side on a server. And if you install SharePoint Designer 64-bit on a server which has installed 32-bit MS Office, then it won’t give good result. More about 32bit vs 64bit architecture decisions in office 2010 can be read at http://blogs.technet.com/b/office2010/archive/2010/02/23/understanding-64-bit-office.aspx.
So, pay attention whilst installing the SPD and install everything of same version only.