In the most general sense, the “loophole” is plain old antisemtism. But I imagine you want something more specific.
In order to be nominated for membership in UN bodies like the Security Council or the Economic and Social Council, the nomination must come from a regional group. Israel’s most natural regional group, the Asian Group, has been blocked by the Arab states. In recent years there has been a move to get Israel into the Western European and Others Group, which would allow it to become a member. A number of the European countries have been dragging their feet on this issue, despite pressure from the US.
Here is a quote from a Jerusalem Post story in 1999: