User blog comment:Kirkburn/Technical Update: April 13, 2015/@comment-452-20150413202500/@comment-24739709-20150414000625

Hey 452! That's actually intentional. We can see the breakdown of which email recipients use clients that can't view rich text, and it was very small - a fraction of a percentage point. Given that plaintext would entail extra work down the line, we decided not to support it. Sorry about that!