They wanted to sell because he stupidly offered a huge amount in a binding agreement. Musk had already bought a portion of Twitter. He wanted to drive up the price with a fake offer, then find a “problem” so he could sell at the higher price. It was just a pump and dump scheme.
The problem is, it’s very hard (and illegal) to do that with someone else’s company. Twitter’s lawyers wrote the purchase agreement pretty well. They basically forced Musk to either buy the company 100% or go to jail, courtesy of the SEC.