It's almost always chosen for density in scifi- most early railguns used tungsten because the density means you can have a very small cross section per unit mas

It's almost always chosen for density in scifi- most early railguns used tungsten because the density means you can have a very small cross section per unit mass, meaning low wind resistance which is important at super and hypervelocity. Of course it's irrelevant in space, but it carried over to sci-fi anyway.