If I understand correctly, Phil started writing his series first. That tutorial prompted several people to start working on their own hobby OS, including Steve's and another one here: http://www.randomhacks.net/bare-metal-rust/. I think Redox was already underway when Phil started writing, but I may be mixing up my timelines.

Edit: There is also http://www.alexeyshmalko.com/2015/bkernel-a-rust-operating-s...

Phil has stated his series is basically for fun and learning. I imagine Steve's and Eric's are similar. You don't necessarily want to "combine effort" with someone when you are exploring ideas on your own.