Two reasons. First, the 5 digits are in the very recent past, most forum members own these in significant numbers more than any 4 digit or 6 digit reference.
Second, the Sport 6's look too bulky to me, in fact, when I wear my 1016 or 1601 for a week and then put my 16610 back on it feels gigantic, couldn't imagine anything bigger. The trend for humongous, oversized watches is nearing its end and the boxy Sport 6's are going to be forever associated with that short trend and not in a good way.