VBx was one of the four language implementations that were originally driving the design of the DLR, the other three being IronPython (from which the DLR was extracted in the first place), IronRuby and Managed JavaScript.
VBx was intended to be a dynamic version of Visual Basic.NET or a .NET version of Visual Basic 6, whichever way you want to look at it. It was primarily intended as a testbed for the flexibility of the DLR, secondarily as a testbed for a more dynamic version of VB.NET and tertiarily (is that a word?) as a scripting language for Silverlight. It has fulfilled its primary goal and the designers of VB.NET decided that they want to pursue goals two and three within the existing VB.NET language, rather than producing yet another slightly different incompatible version of VB. (You already have VB.NET, VB, VBA and VBScript.)
Managed JavaScript was also abandoned, the team was folded into the IE team and is responsible for the ECMAScript 5 engine in IE 9.