Supporting TPL Dataflow on Windows Phone is on our backlog. The reason it's not supported is because one of its dependencies hasn't been implemented for Windows Phone yet (namely concurrent collections). However, we believe we could either remove this dependency or port concurrent collections as well.
I've filed a user voice item for it. Please share & vote -- it helps us to prioritize this work.
Update
I’m happy to announce that we've released a preview that includes support for Windows Phone 8.