Hi!
The cause for RPC-style web services to use the type attribute is that operations in this type of web services quite often represent functions, which often have parameters of primitive types, like
string, integer, float etc. All these "primitive types" are defined in the XML schema language and can be referred to using the type attribute.
Document-style web services, on the other hand, take one single document as a parameter of an operation. Such a document is typically a complex data structure, defined as a global element in an XML schema. Thus, it is referred to using the element attribute.
I also suspect that this distinction may have been made, at least partly, in order to make it easier to tell Document- and RPC-style web services apart by just looking at the messages passed to/from the service.
Best wishes!