De-icing at the gate means wasted resources (gate space not available for another plane). Deicing pads, such as at DEN, that the plane passes through along the way to the runway allow for easy collection for recycling and less elapsed time until takeoff.

When too much time passes after being de-iced, the plane has to be de-iced again. USAir 405 crashed after takeoff from LGA for that reason.

https://en.wikipedia.org/wiki/USAir_Flight_405