These are my personal guesses:

• NBC wanted someone who was the opposite of Lester, and he was given the decision to either host another show or resign.

• Doing a show five days a week for ten years can really wear down someone, and he decided to host something that didn’t require him to host as often (since Dateline is a weekly show).

• Lester wanted a change (as he has hosted the same show for many years).

• NBC (as a whole) is changing, so they thought they should slowly change their hosts to reflect the political view they want, and why not start with a nightly show.