It's not just resumes -- Word is the default application that many people open up to write just about anything, which leads to people sending out emails that consist of nothing but a Word attachment, and nothing but text inside the Word document. With the advent and popularization of Sharepoint, this will only get worse, as people will think of the Office formats as universal (if they don't already.)

For resumes, I think it's just the groupthink of wanting to use what everyone else uses. Also, PDF wasn't always very well-supported on Windows -- I don't use it anymore, but when I did, my Windows systems were more likely to have Office on them than Acrobat Reader. These days the gap is probably narrower, but people don't change their behavior overnight.

I thought it also might have something to do with resume scanning systems being optimized for Word, but that may not be the case.