The key to Azys Lla

[kujoestars]

If you do the Sea of Clouds part after the first Western Highlands section, then you're immediately getting dragged from that into Alphinaud and Tataru's trial, and from there straight into rescuing Raubahn, and then into tracking down Ysayle and heading off to find Hraesvelgr (which directly includes facing Ravana). You never really have time to go back to Bismark this way, so it's somewhat more understandable.

Of course, if you do the Sea of Clouds part first, then it's a bit looser. It could sorta be justified in Bismark being in a far less populated area than most other primals we've faced up to that point (all of whom were within spitting distance of a major Eorzean city), or in just not having the means to fight Bismark available (for Ravana, we pretty much just walk in and pick a fight).

We literally had no means of catching Bismarck until we befriended a Vanu Vanu tribe and got their permission to use an island as bait because unlike the other primals, he doesn't stay in any one location and just flies around and periodically eats an island.