The article I linked explains it pretty well. A small but vocal minority of users wanted more power. MS gave it to them in VB.NET, and with that power came complexity. The huge majority of people who just wanted to do simple CRUD stuff stuck with VB6 (which is supported to this day).
MS killed it in favour of a 'real' programming language. See eg http://msdn.microsoft.com/en-us/magazine/jj133828.aspx
> Looking at the prior art of tools that have succeeded at letting end-users build programs is probably a good bet
Indeed. We have a 50 odd page google doc full of exactly that :)