Because he bought it with the assumption that his offer would be accepted and he could improve the company. Rejecting his offer opens the door to a world in which Twitter is worth less than what he paid for it: from both Musk's perspective, as well as that of others.
It depends on why the offer is rejected. You can say "we think its worth more than that" but if your stock is trading $30 down afterward your investors might have an interesting case about your fiduciary responsibility to them.
