It is a well known fact that uttar pradesh has a very delicate caste composition and BJP has been under tremendous pressure to hit the right combination and satisfy their cadre, RSS and the population of UP. Its my belief that there are some major reasons that led Yogi Adityanath to be the CM-
Perseverance and previous work: Yogi was offered a position in the cabinet by the PM twice but he turned down both the time. Yogi was determined to serve his karmabhoomi and become the CM of UP by hook or crook. He has worked closely with amit shah since 2012 to deliver the state of UP to BJP.
RSS connection: Tired of being sidelined by Modi/Shah duo RSS has chosen an apt time to flex their muscle. His deep rooted hindutva beliefs are equally matched by his actions.He is considered close to RSS and BJP might have buckled under the pressure from Sangh and its cadre.
Hindutva image(HINDU MUTT AND THE GORAKHNATH MUTT): Adityanath has built a huge grassroots organization in eastern UP where he has been a 5 times MP. It indicates his popularity among hindus and keeping in mind the elections of lok sabha 2019 to woo hindu voters BJP has choosen Yogi as the CM.
Age: For someone who has a 5 time MP Yogi is very young and currently at 44 he has a long innings to play in Indian Politics and a big state like UP needs a leader who is energetic and bold.
Seasoned leaders out on national duty: A leader like Rajnath singh, who has an immensely important Home ministry to handle, couldn’t be called for the equally demanding post of CM.With Manohar Parrikar already giving up defense ministry. So out of the promonent emerging state level ministers, Yogi is clearly most popular and that is why he has been choosen as the CM .