Not Enough Time
Earlier, we talked about how it would’ve been nearly impossible for Jack and Rose to escape the rising water levels. Please allow us to elaborate. In this particular scene, Jack and Rose are trapped behind a metal gate with seemingly no way to get out.
If you recall before this, a wave of freezing water takes out a father and a kid, whom Jack just a few seconds prior. This wave, combined with several minutes of being submerged in water, would’ve made it impossible for his hands to fumble around with the gate key. So, yeah, he and Rose should’ve died together earlier in the movie.