When the Emperor, Vader, Empire, and who knows how many dark side acolytes (including Force-wielding Inquisitors, apparently) were against them. Whereas Luke apparently went into hiding a decade or so before the First Order killed off one star system (and somehow immediately dominated the galaxy that simply and easily)? Also, Yoda was freaking old. You mean the guy who thought it was a great idea to have Anakin build C-3PO, and then turn from wanting to arrest Palps to murdering kids in the course of a single evening? Yeah, I wouldn't have wanted him to write the ST either.