Resharper has been in development far longer than Roslyn and seems to be much more suited to working with partially incorrect code (the kind you have while typing away) and incremental changes.
Does Roslyn have the same support?
Does Roslyn have the same support?
I guess JetBrains is heavily invested in ReSharper (and the underlying AST code) which would explain why they won't want to switch right away, if at all.