Why does reducing bike weight have such an impact on speeds?
The two major "drags" on a bike, on level ground, are the rolling resistance of the tires and the wind resistance of the bike and rider.

In general, up to a point, a tire with higher pressure will have lower rolling resistance, as will a tire with a smoother tread.  Tire width also figures in, but mostly in terms of how it influences pressure and tread.

Wind resistance is affected by tire width, number of spokes, tube size, and rider position.  Generally, as the "raciness" of a bike increases, tire width and spoke count decrease and rider position becomes more prone (and hence more aerodynamic).  Tube size doesn't tend to vary much (and may actually increase as you go steel->aluminum->carbon) but that effect is slight and subtle differences in frame design can be more significant.

There may also be ergonomic changes -- eg, the more prone riding position, while more fatiguing, is often somewhat better for peak power output.

Bearing quality hardly figures in at all.  Even cheapo bearings have very low losses, and the difference in drag between them and top-of-the-line bearings is not likely to be measurable.  (The bigger difference is in durability.)

Bike weight only comes into the equation when one must "climb" (and to a very minor degree for rolling resistance), but of course it's rare to have a route that does not go up/down by 0.5% or so most of the time.  That 0.5% increases the power required to hold a steady speed by about 10% over a dead-level course.  But the 3KG difference between the above two bikes would amount to about a 0.4% power difference on that 0.5% slope.