I wonder if a concern they had was Heroku already moving in the direction of node.js support.
However, Joyent really has a leg up on node.js hosting, and seeing as they employ Ryan Dahl and just basically bought node.js, it could be difficult to gain market share, but I believe there is room for others to play.
http://blog.heroku.com/archives/2010/9/20/an_update_on_herok...