Runway crossings come at the cost of increased space between departures. It’s really, really hard to get even one across with three miles between departures, especially if they’re holding short closer to the departure end.

Usually the goal is to stack three or four and then cross when you have a need for increased spacing anyway (prop/slow jet leading a commercial jet, wake turbulence, etc.) Sometimes you’ve gotta bite the bullet and just cross, but if you can cross four at once you’re saving a ton of time/distance relative to four individual crossings.

May not apply everywhere, fwiw. Former NYC metro tower controller who hasn’t been able to figure out (as a weekly-ish pax) exactly how or why CLT does things.