The hexagram had already become a symbol of the Zionist community far before the actual establishment of Israel. It had been adopted at the First Zionist Congress in Basle in 1897, when it was placed on the flag they adopted. When the Declaration of Independence of the State of Israel was made, the flag and symbol had already become well-ingrained into Zionist and Israeli thought. It was hoisted up by crowds in celebration after the UN partition plan was passed in the General Assembly on November 29, 1947, and when the declaration of independence was announced, it was done so on a dais with a picture of Theodor Herzl (founder of political Zionism) flanked by the flag on both sides.
There was a brief moment where the flag was almost abandoned as a symbol, but ultimately the decision was made to keep it. This was debated only a few days before the declaration of independence on May 14, 1948.
Some motivations for using the symbol:
It would provide a symbol to rally around, similar to the Christian cross.
It was Jewish, but evoked no clear-cut religious associations, meaning it didn't threaten the perception of Israel as a secularist-led state.
It was used around the world by Jewish communities to rally around, both simple and used by Jewry on the whole to signify themselves.
However, there were those who claimed the flag should be exchanged, to differentiate between the Zionist movement and the Jewish state by flag.
Minister of Foreign Affairs Moshe Shertok (Sharett) explained that this was desirable:
...so as to avoid complications for Jewish communities when they raise the international flag of the Jewish people, namely the Zionist flag, and misunderstandings may occur, or the impression might be that they are flying the flag of a state of which they are not citizens.
So a competition was organized for a new flag. Some new ideas were considered, including this one with more on the flag, and this one using a different Jewish symbol.
Ultimately, after Sharett discussed with the leaders of Diaspora Jewry (Jews outside of Israel), the debate was put to rest. Rabbi Silver in New York said:
We would prefer to leave the Zionist flag as the national flag of Israel, with a minimum of changes. We feel that the fear of complications as a result of use of the flag at Zionist gatherings overseas has been somewhat exaggerated.
The question of dual loyalty now removed, the debate was finally decided once and for all on October 28, 1948, when the Provisional Council of State voted to adopt the Zionist flag as the flag of Israel. The resolution went into effect two weeks later.
