The gerus bill would decentralize gerus, allowing local batei din (outside of the Rabbanut) to process gerim. This is how it used to be anyway, so it’s not like it’s a fundamental change. It would also benefit chassidim and other groups who prefer to use their own rabbis instead of going through the bureaucratic channels of the Rabbanut.
The left likes to make changes that would make gerus less hard because of 1) universalism — those on the left are more likely to see Yiddishkeit as something of more universal appeal to all of humanity, rather than the teachings of an insular minority that doesn’t care about the world, 2) sympathy for left-wing religious rulings (such as those of R’ Uziel and others) that allow gerus to be completed even without the ger becoming completely observant, 3) a desire to make life easier for Russian immigrants who aren’t halachically Jewish but have Jewish ancestry and a Jewish identity and served in the IDF, 4) a desire to undercut the increasing control of the Rabbanut (and thus Israeli society, in some respects) by the charedim.