Because think of how they viewed the world back then.

It was largely 'across' as they crossed the ponds from one continent to another.

Nowadays we think more 3 dimensionally, less linear, due to how travel and communication is conducted.

They looked at lots of 2d maps, with a point a on one side, and b on the other.

Now we grew up with globes and google earth.

2000 years ago they the entire earth was roman empire.

300 years ago, they had maps and rich people had globes.

It's more of a sign of change in the way we think.