I think one of the reasons they may have stopped is because they ran into some problem calculating what a "day" actually stood for.
In Genesis each "day" is basically "evening" until "morning" as in Genesis 1.5 which states "and there was evening and there was morning, a first day."
It doesn't even say if this was a 12-hour day. 12-hour days only occur at certain times of the year from certain places on the surface of the globe. According to Jewish tradition, this was Rosh Hashanah: autumn, and there are no 12-hour days in the autumn from evening to morning.
To add to this paradox the Hebrews did not believe that the earth was a sphere in Genesis chapter one but a flat platter over which a half bowl was tossed over, the firmament, upon which the sun, moon, and stars were affixed. This firmament kept out the heavenly or cosmos of waters from falling upon the earth (the dividing of the waters above from the waters below was established by the firmament which was named "sky", remember?)...
So there was no globe or hemisphere over which to pinpoint a spot for seasons for 12 or 24-hour days to calculate and divide the years to start the march of prophetic years the Millerites had divised and that Franz had pulled out of the old hat to come up with 1975.
Like the Mormons have done with many of their old teachings and some of the failed prophecies, the Watchtower stops talking about things and hopes that people forget. The less people talk about stuff, the less people remember. The less they remember, the less it becomes considers official teaching.
That's the way it works in Mormonland. And that's the way 1975 has been working in Watchtowerland too.
If you try to dig it up, people shrug. And it doesn't work when you compare it to both critical academic theology and tradition teachings. Watchtower likes it that way. They learned this little trick from the Mormon leadership.
So you won't hear anymore speculations about Biblical creative days anytime soon...or probably ever again.
