Something to consider...the modern scene divisions that we're so used to are just that - modern, or at least more modern than Shakespeare's time. From what I've learned in the acting class I'm taking (if I have my facts straight - it's early yet), "scenes" to Shakespeare were either when the stage was completely clear, or when there was any entrance or exit (now we call those "French scenes"). The link below gives an article about act and scene divisions from the Oxford Companion to Shakespeare, which mentions that even the divisions between acts changed from Shakespeare's earlier plays to his later plays, probably as a result of his later plays being performed indoors (they had to trim candle wicks periodically to keep them from smoking, so took breaks for that).
Also, the earlier quartos of his plays do not have scene breaks. This appears to have been added to some of the plays in the First Folio (which, of course, was published after Shakespeare's death), but not even all of them.
BUT...all that having been said...why, then, did later editors make it such a long scene? #2-clane hit it on the head, in my opinion - everything in that scene is so interconnected that it would not have flowed as well to chop it up. We also see Polonius being mocked in two separate parts of it ("fishmonger" and then later Hamlet mocks him with the players), so it seems like it's the comic relief bit.