Bennett is the leader of a party that ran on an Alt-Right ticket. In fact, the literal translation for his party's name "Yemina" is "Rightwards".
In addition, he promised several times during his campaign that he will never ever form a joint government with the left wing parties. He even signed it on a document on national TV. See; https://israelheadlinenews.com/saar-says-open-to-power-sharing-deal-with-lapid-or-bennett/https://www.93fm.co.il/radio/677589/?utm_source=site&utm_medium=search
Here is the video of signing the document on TV, sorry but it's in Hebrew; https://www.youtube.com/watch?v=os_CJSiPH4U
However, in order to persuade him to put his reputation at extreme risk and severely dissappoint his supportersseverely dissappoint his supporters (according to recent polls some 60-70% of his voters say that they won't vote for him again) - he had to be given the biggest prize.
Most of my sources are in Hebrew, will look for sources in English and post them later.