Maybe because Christian Freeling, the inventor of Grand Chess, felt that the possibility of placing a further powerful piece on the board would be too much! Pawns can promote as early as the 8th or 9th rank. The rule also means you don't need to have spare pieces, and it probably makes quite good sense.
Grand Chess is an chess variant played on a 10 x 10 board. there are 2 more added pieces called cardinal(Archbishop) and Marshall(chancellor). I got a question about its promotion rule, Unlike orthodox chess, A promoted pawns can only be replaced by any previously captured piece of the same color. If no captured pieces are available to replace the pawn, then the pawn must stay on the 9th row. Why grand chess applies this rule. I wonder the reason and want to listen your opinion.