Mourning on Yom Ha-Atzmaut

Q: Why doesn't Yom Ha-Atzmaut cancel out the mourning practices of "shiva"? While it is not a Torah-ordained holiday, it is known that when there is a conflict between personal mourning and communal joy of Klal Yisrael – the joy of Klal Yisrael takes precedence.
A: Regarding its laws, Yom Ha-Atzmaut is like Chanukah and Purim. But regarding the laws of mourning, the Shulchan Aruch (Orach Chaim 696:4) states that Chanukah does not override the laws of mourning, while Purim partially overrides the laws of mourning. The reason for this is that Purim is a decree of the Prophets, while Chanukah is a Rabbinic decree. Since Yom Ha-Atzmaut is also a Rabbinic decree, it is similar to Chanukah and does not override the laws of mourning.