The question implies:

1. that the maximum flying speed is mainly affected by the compressibility drag and;
2. that compressibility drag is mainly related to sweep angle;

but this is not 100% correct.

Maximum flying speed depends on the equilibrium between thrust supplied by the engines and drag generated by the aircraft.

Thrust obviously depends on the technological development of the engines (which was for sure different between the first version of the B747 and the first version of the A380) and, among other factors, on temperature, density, humidity, height, ...

Drag depends as well on the technological development of (the surface of) the aircraft: its shape, how much surface has a turbolent boundary layer, surface finishing, eventual use of area-rule, type of airfoil, ... And (induced) drag depends on lift as well.

So, maximum flying speed is affected by a lot of parameters and not only by compressibility effects. And upon this, also structural and/or aeroelastic (flutter) limitations can restrict maximum speed.

Compressibility drag is not influenced by the sweep angle alone.

If for example the area-rule concept has been used in the design phase then the geometry of the whole aircraft is important. Area-rule is a particular smooth variation of the sectional area of the aircraft which minimises compressibility drag at transonic speed. Sectional area is the area that you get when slicing the aircraft longitudinally.

Another way to reduce compressibility drag is the use of supercritical airfoils, which have been for sure employed on the A380, not sure on the B747.

So, sweep angle is not the only parameter influencing the value of maximum speed or compressibility drag and its value is actually chosen as a compromise of many other (contrasting) requirements.

Side note about sweep angle: the point on the airfoil where the highest speeds are reached is around where its thickness is the highest. For a jetliner this point is important because it is the point where speed can reach transonic or supersonic speed with a relevant increase in drag. For this reason the real sweep angle to be considered for aerodynamic considerations is not the one at the leading edge rather the one passing through the highest thicknesses of the wing.

The question implies:

1. that the maximum flying speed is mainly affected by the compressibility drag and;
2. that compressibility drag is mainly related to sweep angle;

but this is not the whole story.

why does B747 cruise at a lower speed than A380?

Maximum flying speed depends on the equilibrium between thrust supplied by the engines and drag generated by the aircraft.

Thrust obviously depends on the technological development of the engines (which was for sure different between the first version of the B747 and the first version of the A380) and, among other factors, on temperature, density, humidity, height, ...

Drag depends as well on the technological development of (the surface of) the aircraft: its shape, how much surface can maintain a laminar boundary layer, surface finishing, eventual use of area-rule, type of airfoil used for the wing, ... Plus (induced) drag depends on lift as well.

So, maximum flying speed is affected by a lot of parameters and not only by compressibility. And upon this, also structural and/or aeroelastic (aka flutter) limitations can restrict maximum speed.

the main reason for sweep is to delay compressibility effects.

Compressibility drag is not influenced by the sweep angle alone.

If for example the area-rule concept has been used in the design phase then the geometry of the whole aircraft is important (area-rule is a particular smooth variation of the sectional area of the aircraft which minimises compressibility drag at transonic speed. Sectional area is the area that you get when slicing the aircraft longitudinally).

Another way to reduce compressibility drag is the use of supercritical airfoils, which have been for sure employed on the A380, not sure on the B747.

So, sweep angle is not the only parameter influencing the value of maximum speed or compressibility drag and its value is actually chosen as a compromise of many other (contrasting) requirements.

Bonus material about sweep angle: the point on the airfoil where the highest speeds are reached is around where its thickness is the highest. For a jetliner this point is important because it is the point where speed can reach transonic or supersonic speed with a relevant increase in drag. For this reason the real sweep angle to be considered for aerodynamic considerations is not the one at the leading edge rather the one passing through the highest thicknesses of the wing.