The diurnal bulge in the atmosphere plays a rather minor role during launch. Launch is short in duration and involves an aerodynamically shaped vehicle that punches through the thickest part of the atmosphere. Whether its locally 2 PM (bulge is thickest) or 4 AM (bulge is thinnest), the density at orbit insertion altitude is tiny compared to that of the troposphere, stratosphere, and mesosphere through which the vehicle has plowed before getting to that orbit insertion altitude.
The diurnal bulge plays a significant role on a satellite in low Earth orbit. The very short duration launch interval (ten minute or less) becomes years of operations for a satellite in low Earth orbit. The effects of drag pile up over the years. On top of that, most satellites have anything but a nice aerodynamic shape. One of the first things a satellite does after separating from the launch vehicle is to deploy its solar arrays.
Another big factor is the Sun. One solar flare, and poof! the upper atmosphere swells. Drag on a satellite can easily more than double between solar minimum and solar maximum.