As someone who maintains a vimscript plugin... HAHAHAHAHAHAHA
There's API differences between vim8 and neovim that make it a pain to do certain tasks on both (UI manipulation or anything with asynchronous work). Supporting both in the same plugin is annoying. The split is already there and it's only going to get worse, so might as well go with a language that doesn't suck to write. I think upstream vim is the one needing to catch up. Vimscript sucks to write and maintain.
There's API differences between vim8 and neovim that make it a pain to do certain tasks on both (UI manipulation or anything with asynchronous work). Supporting both in the same plugin is annoying. The split is already there and it's only going to get worse, so might as well go with a language that doesn't suck to write. I think upstream vim is the one needing to catch up. Vimscript sucks to write and maintain.