It’s pushed off to prevent chillul Shabbos. Yom Hazikaron was supposed to be Motzei Shabbos/Sunday, but they were worried that that could lead to Chillul Shabbos due to the preparations for Yom Hazikaron which start a few hours beforehand. Since Yom Hazikaron was pushed off to Sunday night/Monday, Yom HaAtzmaut had to be pushed off to Sunday night/Monday.
They did the same thing with Yom HaShoah last week.