That’s because a lot of Chareidim (Eidah, Brisk, Satmar, etc.) , b’shitta, hold it is assur to vote and therefore don’t vote.
And even though Chareidim are an increasing percentage of Israeli Jews, considering the influx of Russian goyim (some of whom go through a fake conversion but most of whom don’t even do that much) voting for parties like Lieberman’s Betanyahu (now merged with Likud), it dilutes the true Jews vote.