The poison pill is to make it so that the person can't ever get 51% because it's not good for everyone else.
The 'short answer' to the question of 'why no poison pill' is simply because the Board reached some kind of agreement with Musk.
I like Musk when he's in his lane, I don't see any good coming from this.
Everyone is nuts to talk about 'shareholders' - who cares? As a 'consumer' - I want a good service and cheap (i.e. free) with no ads. I have zero interest in 'shareholders' of a company I'm not a shareholder in, other than legal protections.
Caring about 'shareholder value' while not a shareholder, is like caring about some rich guys bank account - when often it's a zero sum game.
Every dollar in an investors pocket should be a dollar on your pocket in term of reduced price etc..
So aside from some things he may be able to do t bring the share price up ... I'm wary that much at all will actually benefit Twitter, and he could ruin it.
I don't personally care about it, but it's an important media tool - it's how a lot of information gets out.