The value of Twitter is implicit to sales and marketing professionals. Everyone knows that it is important in some respect and that they should probably be using it more than they do but are not really sure how or why.
By contrast, the value of LinkedIn is much more explicit. If you asked someone in B2B sales what they thought was valuable about LinkedIn, they would probably tell you that “it is an amazing tool for prospecting, nurturing leads, researching, etc.” The same goes for marketers, recruiters, business owners and so on. All of them would give you different but similar answers about why LinkedIn is a valuable tool for them.