A cancellation machine cancels the stamps. It bends the envelope as it is routed through various chutes and machines. I believe rollers move it along - rollers than pinch the envelope and put pressure on its contents. With normal mail, this isn't a problem - paper bends and withstands crushing pressure. Chips, however, might be sensitive to those pressures. In addition, the way the envelopes are pushed through narrow chutes, it offers an opportunity for chips or other irregular contents to be squished through the end of the envelope, spilling the contents.

My assumptions are based on the machinery the post office used in the 1970s; I don't know whether they have more sensitive machinery now (I have my doubts).