User blog comment:Kogath/Monster Hunter X Discussion/@comment-25706585-20151005104307/@comment-6050823-20151006220911

Abyssal Lagiacrus move to the sea and continue to live in the sea for the rest of their life. The reason for this is because the Abyssal Lagiacrus can't move on land as well as Lagiacrus/Ivory Lagiacrus from it being bigger and heavier, hindering it on land.

Now Capcom could make an explanation say that the Lagiacrus we encounter in MHX are some kind of subspecies like many other monsters such as the Kushala Daora, Chameleos, and Raths.