Probably because of the R1S having a 14.9 inch shorter wheelbase than the R1T.Personally I have never understood why Rivian is unable to install the 180 pack in the R1S with a folding third row. It seems to me the usable floor space is the same at the third row as at the second row. See the diagram below:
Of course this diagram is not to scale, but it demonstrates to me there is room for a folding third row, as long as the seats are able to fold down as low as the second row folds. My opinion is ignorant, though, because there is obviously a reason why they removed the 180 pack from R1S marketing material. Maybe they will find a way to fit the pack in, but from what they have released so far, it seems they are not sure it can be done.
My assumption has been the R1S max pack will be more than 135, but less than 180. Time will tell if that assumption is correct or not. I hope Rivian finds a way to make the 180 work, because a lot of people will be disappointed if they can't. Including me. Although I am leaning toward the R1T 180 pack version, because it will have the ability to add the Aux battery module shown in the drawing above. I doubt the R1S will ever have the Aux pack option.