The reasons seem obvious (see bolded print):
Prime Minister Benjamin Netanyahu and Foreign Minister Avigdor Liberman on Thursday night made official the news of a mergerbetween their respective Knesset factions, Likud and Yisrael Beytenu, saying the new right-wing super-faction would improve governability in Israel and allow them to tackle burgeoning internal and external challenges facing the country.
“A joining of forces will give us the strength to defend Israel from military threats, and the strength to spearhead social and economic changes in the country,” Netanyahu said, standing alongside Liberman in an address to the media
“We will ask the public for the mandate to lead the State of Israel with strength in the coming years. This will greatly strengthen the government, the prime minister, and the country,” Netanyahu added. “A clear mandate will allow me to focus on what’s really important.”
Netanyahu cited the Iranian threat and Israel’s rising cost of living among the challenges that would have to be faced by his government in an unequivocal, determined manner.
Speaking after the prime minister, Liberman said: “The joining of our forces presents a combination, as you said, of strength and unity. This is what the residents of Israel are expecting today…. We spoke a lot about government reform, and in effect, today marks the beginning of true government reform.”
The merger has little political downside, as it should not arouse dissatisfaction amongst supporters of either the Likud or Yisrael Beyteinu. Lieberman is well-regarded in his former party and never lost contact with it. Some may even believe that Lieberman provides a toughness that Netanyahu lacks. Lieberman's core constituency- immigrants from the former Soviet Union – are gratified that now one of theirs has a shot at becoming prime minister.
Netanyahu may also be counting on the fact that the left will not be able to match the merger. Who is going to yield to whom is a question that already dominates conversation on the left.
We will know only on January 22, or in the wee hours of January 23, whether the surprise electoral mergerbetween Yisrael Beitenu and the Likud paid off politically.