mourning aveilus - Meat on Rosh Chodesh of 9 days

Why is The reasoning that Sefardim do eat on Rosh Chodesh and Ashkinazim do not?

Sefardim

Is It Permissible To Have Left Over Meat From Shabbat During The 9 Days

Ashkenazim

EATING MEAT AND DRINKING WINE DURING THE NINE DAYS

The Nine Days - Laundry, etc.

Answer

I found it on the Lakewood scoop: The reasoning for Ashkenazim is:

The Maharil and the Seder HaYom (from the times of the Arizal) both say that we do not eat meat even for the Seudas Rosh Chodesh. The reason, says the Seder HaYom is that we eat meat on Rosh Chodesh because, as the first day of the new month, it is a day of Simcha. Not so Rosh Chodesh Av which is a month of Puranus, troubles. Also it is the day that Aharon HaKohen was niftar. This is also the opinion of the Arizal in Shaar HaKavanos.

Summary; Askenazim focus on the fact that normally Rosh Chodesh is a simcha but here it is the begining of a bad thing so no Simcha, Plus it is Ahron Yahrtzeit which ussaly calls for a fast.

The reasoning for Sfardim is:

The Shiyarei Knesess HeGedola writes that the minhag in Turkey, at the time, was to eat meat and drink wine during the Rosh Chodesh Seudah, even in Av. This is also the opinion of the Chida. The Toras Chaim says that if it is your minhag to consistently eat a celebratory seudah with meat and wine each Rosh Chodesh, you should do so on Rosh Chodesh Av as well. The Darkei Chaim V’Shalom takes this a step further and says you should only eat meat if you are noheg like the Talmidei Baal Shem Tov and make a public seudah every Rosh Chodesh. (See Minhag Yisroel Torah 551:6)

Summary: Sfardim and it seems Chassidim too celebrate regularly and do not focus on the negative aspect of the month because Rosh Chodesh is still a simcha. I guess it is consistent with them in genral being more in tune with Rosh Chodesh.