Leaving the option of gaining control via 51% hostile takeover would essentially be a ~50% discount for Elon. If your goal is to sell at the highest price you don't want to count on someones word or intentions.
Plus even if Elon really wants all 100% for reasons, the bank providing funding would certainly want him to consider any cheaper options since it's (nearly) the same gain for them but less risk to finance buying 51% vs 100%.
There was nothing stopping him from increasing his share while the board was deliberating. He could have used that tactic to increase pressure. The poison pill clause meant that the board was free to take whatever decision they wanted in the timeframe they deemed fit.
