Kaido explained that an awakening happens when the devil fruit user’s mind and body catches up to the goal of the devil fruit. In Egghead island we learn that each devil fruit was created with a wish. Kaido was only partially awakened. My assumption is the wish of Kaido’s devil fruit was to be invincible. Kaido’s body was caught up to the wish of his devil fruit, but in his mind he was still hoping someone would defeat him. The wish of Luffy’s fruit is to be free. In his mind, he already had freedom, which is why we can see the cloud effects surrounding his gear 4th forms, which are his partially awakened forms. His body, however, was not synced with his mind, so he had a time limit on his semi-awakened forms. When Luffy was killed by Kaido (yes, he was literally killed) his body got what it needed to awaken. Gear 5’s heartbeat seems to be what activates the form. Since Luffy never died before, and his heart never stopped beating before, him being killed essentially served as a “reboot” button for his heartbeat. So, Luffy would have never acquired gear 5 if he never died.