While we do want to get there, there are a couple things currently holding us back. First, the older WinForms implementation of the .NET Languages Add-on isn't nearly as powerful as the one in our newer WPF .NET Languages Add-on (whose codebase is also used in the Silverlight and UWP versions). That particular implementation has a much better design and can be enhanced with language updates much easier and reliably. In fact, we are adding C# 6 and VB 14 support to that add-on in the upcoming maintenance release. We want to get that design backported to WinForms as well, but in order to do that, we need to backport the entire newer API model of SyntaxEditor in the other platforms. A stumbling block there is how WinForms renders UI (GDI-based) compared to the other platforms that are XAML based. Yet we are hoping to look into it again soon here because it would be ideal if we could have the WinForms SyntaxEditor in sync feature-wise with all the other SyntaxEditor offerings.
The other major issue and the main reason we don't have C# 7 and VB 15 support in the WPF version yet is that Microsoft has done a horrible job after C# 5 (when they open sourced Roslyn) of updating their specs. Without specs, it's difficult to know what parser grammar updates to make. They only just recently released a C# 6 unofficial spec and the VB one is still on VB 10. That has been very frustrating. I highly encourage you to support and comment any issues in their Roslyn GitHub repo related to releasing updated language grammar specifications that match what they have officially released.