It all has to do the sensitivity of the detection circuit and how brutally the unit is designed to open and close. With hardware you have to be able to fashion your slope in both directions and to do this with minimal obvious effect. I cant imagine that with software that this isnt really easily adjustable.

But the other side of it is how the engineer perceives the sound of the source being gated and how its interplay with the music is effected.

This is the main thing above all else. Gates are very powerful tools and can ruin a mix without a lot of effort.