The main reason for using is that the applications developed are pretty less in size.
Any application developed with .Net will require, quite understandably, the .Net framework, the installer of which is way too big and the actual installation is also quite large.
With VB 6.0 ( shall we call i legacy now?) a couple of essential files copied and registered on the client machine and you are done. The actual package is too small and easy to distribute, say over the internet.

The application developed with VB 6 are less resource intensive, at least that is my observation.
My personal experience with VB.Net is bad, it is no-where like VB6 and is a trap laid my MS for VB6 programmers. I believe that C# is far more easier and the mindest is also set that we are doing something new. It will be interesting to see how may VB6 programmers prefer VB.Net, i went for C#.