I can't find any official statements about this renaming (there's a BigBreak branch which is supposed to take care of renaming as per a thread reply on the forum[1]) but I assume that people have been citing the name as a barrier to a widespread adoption of Nimrod [2][3].

[1] http://forum.nimrod-lang.org/t/541

[2] https://news.ycombinator.com/item?id=7076451

[3] https://news.ycombinator.com/item?id=7074774