From https://www.mediawiki.org/wiki/Help:VisualEditor/FAQ:

Will the visual editor be enabled on talk pages?
​No. This question comes up quite often.

• The visual editor is designed to edit content, plain pages of text[1].
  • Talk pages aren't content. Many of the tools and design patterns that make VE nice to use to edit content make it poor to use for discussions. ​
  • To make it usable for discussions, we would have to remove or break many of those patterns in VE. We have spent a lot of time researching with users what works best there.
• VE can't deal with structured discussions and plain-text discussions are not structured discussions.
  • Discussions like they are on traditional talk pages are not structured discussions from a technical perspective, despite the fact there is a certain number of colons or bullet points added to each answer to provide a pseudo-structure. With the current design of classical discussions, a piece of software can't know who has replied to whom – only humans can. There is no real connection between posts (which post is the parent/child of which), which is the definition of a structured discussion.
  • At the moment Flow deals with structured discussions. In Flow, each post is independent with a unique ID, linked to other posts and to a Topic (also with a unique ID), with a specific history, and all posts can be targeted precisely. It would be possible in the future to have conversations at multiple places, to move topics or replies, and to create sub-discussions with Flow. Classical talk pages, using VE or not, do not allow that.