Why are draconian abilities mismatched?

[TyGuy]

Sivaks still can shapeshift to sow confusion, they just do it while they're alive now as a sort of kill-and-replace ability that they can use as a reaction. Which honestly seems more useful/practical?



I imagine the designers thought that the corpse shapeshifting lacked mechanical "combat oomph" in comparison with the other death throes, so they changed the death throes into the explosion and moved the shapeshifting to an ability they can use while they're alive.

I'm looking through old content and sivaks originally, in AD&D 1987, could shapeshift upon slaying a creature, shapeshift when slain, burst into flames when slain by a larger creature.
In 3.5 they maintained all three of these properties but would also burst into flames if slain by something other than a humanoid. Interestingly, female sivak would burst into flame when slain regardless of what killed it.

All in all, 5e dropped the shapeshift when slain. Probably to clean up/ simplify. I still have no idea why they do fire instead of cold. But they're pretty faithful to the original.