They want to limit it to actual Jews:
Israeli politicians have focused on the “grandfather clause” within the Law of Return, which recognizes third-generation Jewish roots as sufficient for Israeli citizenship. This means that a person who has one Jewish grandparent, but who is not considered Jewish under Jewish law, can immigrate to Israel and claim citizenship. The clause also states that the spouse of this person is entitled to citizenship and residency as well.
The ultra-Orthodox parties have proposed that the grandfather clause be scrapped, which would mean that only those with Jewish mothers and fathers qualify for the streamlined path for Israeli citizenship.
According to Jewish law, one is a Jew if one's mother is Jewish. If one's maternal grandmother is Jewish, then one's mother is Jewish and therefore you are Jewish. Being Jewish is NOT passed on by a paternal grandfather.
The reason for the “Jewish grandfather” rule was that it mirrored the Nazi definition of being Jewish. Israel after WWII didn’t want to throw up any roadblocks to Jewish citizenship. They wanted anyone who could be persecuted for being a Jew to be able to emigrate to Israel.
Times have changed. There have been incidents of “Russian Jews” emigrating to Israel and turning out to be neo-nazis. That is one of the main drivers for reforming the right to return.