When Adar Begins...

The Mishnah in Ta'anis (26b) states: "When Av begins, we decrease joy." The Gemara (29a) adds: "Just as we decrease pleasure when Av begins, we increase pleasure when Adar begins." This last phrase has been made into a popular song to the tune of Pick a Bale of Cotton: "mi-she-(mi-she-, mi-she-)nikhnas Adar marbin (marbin, marbin) be-simkhah" (link)

Oddly, though, the Shulchan Arukh (Orach Chaim 551:1) quotes the rule about decreasing joy in Av but not about increasing joy in Adar. I have no idea why this is the case.

[One could jokingly say that this proves that the Shulchan Arukh wants us all to be (stereotypically unemotional) Litvaks. Or the other side can say that he expects us to be Chassidim (who are stereotypically always happy), so there is no need.]

Seriously, I'd appreciate knowing any possible explanations of this omission.