VB6 is long gone (more than 20 years now since Microsoft released a compiler that handled it, and out of support for many years), but VBA is so different to VB.NET that I suspect that they have to keep it for backwards compatibility. The problem is that Office would need to be rewritten into .NET (with the associated slowdowns) in order for a .NET based scripting language to make any sense (in general Win32 apps can only access .NET components if they have been exposed via COM, and these are in the minority)
Biting the hand that feeds IT
