The 81-Square Universe

Why don't the pawns promote?

I don't understand your question. The rules are like that. You could as well ask why the fire demon burns other pieces in Tenjiku Shogi, or why the Lion can capture two pieces in one move, or why the King doesn't promote in most Shogi flavours: It's the rules.

I mean Why are the rules like that?

Seems like a design decision of Mr. Dai-Dai

Yes. Send him an email.
Hm. Wonder if the W3C have got round to finalizing that RFC for time-travel emails yet?

Hi guocuozuoduo
Interesting question, Tai-Shogi has the same pawn properties, but Maka-Dai-Dai pawns do promote.
Dai-Dai, Maka-Dai-Dai and Tai-Shogi were all first mentioned in the same book and therefore developed more than 200 years ago. Maka-Dai-Dai has the least congested initial set-up of these three games.
What all three games have in common regarding promotion is that it is only made by capturing an enemy piece, it is compulsory and it often results in reduced powers.
So why aren't the pawns promoted in Dai-Dai and Tai-Shogi?
One possible reason may have been to avoiding games being exchanged down to a pawn promotion ending.
Or it could have been to avoid giving such a valuable promotion, a gold has 6 times the power of a pawn. Exchanged pieces are often protected by pawn early in a game.
Far less likely is that the rule was inserted by the developers on a whim.
So if either of the first two reasons are true then the question is, why do Maka-Dai-Dai pawns promote.

And why do the King promote in Maka, Dave?

You've asked for that last question, dave

Design issues aren't the same thing as a whim, but that doesn't mean that the rule set documented in the old manuscripts are the only possible way of playing the game. At least into the 18th century in chess it was allowed to rochade out of check, Wa shogi is being played with and without drops (which is also the main difference between Sho Shogi and Modern Shogi) and the fact that different sources state different rules for Tenjiku and Taikyoku (at least) may well reflect different rule sets in different locations.

There may be non-gameplay-related issues as well, like philosophical or historical ones.

And finally, personal preferences come into play as well.

Why is Ko-Shogi played with round pieces instead of angular ones? It's well possible that the sets were made from Go sets (the board being identical), but it's hardly a design requirement. But maybe the designer just liked the idea of placing the pieces onto the lines better.