It was what Twitter was worth on the stock market (substantially less than $44 billion), plus it had lost (I think $2 billion) during the past 2 years, and Elon paid 1/3 more for it than its established worth.

My guess he offered it to stop all arguments because the board didn't want to sell to him. Afterwards, when he wanted to withdraw, a lot of people wanted to accept the offer because it was so much more than the shares were worth.