I asked this question in the past. Once he made a valid offer they had no choice because it was a "public" company. At that point they were kind of stuck. The shareholders profits are what mattered and their charade was up. At least that's about what I got for an answer

Because the offer to buy out Twitter was made to the shareholders, which are different people from the upper brass at Twitter who knew what a bad idea it actually was to their plans. Shareholders just saw a great payout to their investments, and began twisting Twitter’s arm to sell, because otherwise the company’s stock value would tank and the shareholders would’ve been in the whole.

They were trapped in a between a rock and a hard place situation, and a win win for us.