Probably the same as every other attempt at this since the mid 90s. People who can code front-ends don't need such a tool. People who cannot code front-ends hire people who can, and still don't need such a tool.
Not all UI-builders are a fail. Visual Basic, Qt, Delphi, MS Access all had/have some very popular and useful UI-Builder. There is always a middle between no skill and high skill.
Loved Delphi, Forms, Blend, Netbeans Matisse and JSF designer, Blend, Outsystems, ...
As mentioned in another thread, these kind of tools with good workflows and developer experience don't get written for free.