So Twitter was at $70 per share a year ago. So what? Jack Dorsey was CEO a year ago, too. The share price was $33 less than a month ago. But, despite an absolutely incredible roller-coaster news cycle, things have been definitely trending down at Twitter ($33/share last month), which was reflected in its share price. The current executive team (and Dorsey) had wasted time focusing on things that didn't matter instead…
Everyone currently holding $TWTR believes that the true value is greater than the current price. To make a successful hostile bid, you need to pay not just "more than the current price", but "more than the holders of 50% of current shares believe the company to be worth". Usually bidders use analyst price targets to guess at the distribution of holders' internal price targets. This is what makes it so difficult, and…
I can only add one minor point from a general and international perspective: 50% may not always be enough. At least in Sweden I think you need 90% to accept a bid before you can legally force the remaining 10% to sell their shares. Even if you accept that there will be minority shareholders you will need 67% before you can do some things, like e.g. issue new shares without giving minority owners right of first refusal.
I'm sure there's a lot of devils in the details all over the world, and I'd be surprised if there are none in the US.