Because there are no client components to do ODBC. To access a database, you need some components on the PDA. Pervasive doesn't offer those components.
I don't know of any specific tutorials. I created a test back when VS.NET was first released but I can't find it now. As I recall, all I did was create the web service in C# on the Windows host and then a C# PDA application that uses the WebService.