Reloading the rifle is one good reason. The chamber opening of a bottleneck case is much larger than bullet diameter.

Reloading the 450 No2 is like dropping a pop can into a open 55 gal. drum.

Also even though I trim ,if necessary, after each loading, and seat bullets and crimp in two different steps, there seems to be less chance of buckling a case when crimping a bottle neck case.

And if, for no other reason there is a much, much, much larger, and cheaper selection of bullets for the .458 bore.