Its for energy-maneuverability reasons. Like everything else in engineering its a trade off. 1st important fact to understand is this: In airliners, and most previous gen combat aircraft, designs are (primarily) made for flying in level (ie cruise) most effectively. At this optimal cruise point/line, the best engine fuel efficiency and aerodynamic efficiency, the lift/drag ratio, are closely matched. This is the reason why airliners have much higher wingloadings. They are great at cruise, but has to solve the slow speed performance with overly complicated high-lift devices like 3 stage flaps..
4th gen aircraft are designed for preserving energy while turning, so they DO NOT actually cruise at near their best L/D ratio.
Points below are the mathematical part to explain the sentence above, feel free to skip to the next paragraph.. When calculated with data from MiG-29's aerodynamic manual;
-A MiG-29 cruising with full fuel and weapons payload (14900kg) at 6000m (20k feet) altitude at M0,85 requires lift coefficient of Cl=0,145 to stay in the air; that corresponds to overall L/D of 6,5.
-Best L/D MiG-29 is capable of which is 10,5@Cl=0,4.. That lift coefficient equates to MiG-29 pulling 2,76Gs at same flight conditions..
-Even when Cl=0,85 which equates to pulling 5,86Gs, the MiG-29 is more aerodynamically efficient than cruising at same flight conditions.
-If payload or altitude get smaller, necessary Cl will decrease, or in other words, for the same Cl and L/D plane will pull even higher Gs. On extreme example, MiG-29 with 50% fuel at sea level is more aerodynamically efficient while pulling 9Gs at M0,85 than flying straight in level. (This is not to say it requires less thrust overall; it means it requires less thrust per lift generated.)
While all calculations about Cl may seem irrelevant to nose angle, it is not.. Cl is also directly proportional to angle of attack (AOA). Summary from all these calculations is this: irrelevant of weight or flight conditions, if you fly MiG-29 even at 12 deg AOA manuvering, it will be more efficient than flying it straight at just 1 deg AOA.. Point where Mig-29 is most efficient is maneuvering at 4 deg AOA, not cruising at 1 deg. This is a physical necessity for energy efficient maneuvering and applies to planes like F-16 as well.. An F-4E can turn well, but cannot really sustain that turn. These different design choices and optimizations is what "designed for sustaining high G turns" actually mean.
Getting back to Su-27/30.. I think the basic reasoning is quite obvious at this point. Nose of the aircraft doesn't create any meaningful lift, it creates only drag, reducing the L/D, and making the plane less efficient. If you make the nose sit at a certain angle like ~6 degrees in Su-27, it makes aircraft more efficient at around that angle of attack, but its less efficient at the others.. In the most basic sense, by pointing the nose downward, designers trade level flight fuel efficiency and acceleration performance at the higher speed + lower altitude parts of the flight envelope.. At the same time they get better sustained turn performance and better acceleration on the low speed high altitude part of the envelope. MiG-29, F-15 and F-16 also has their nose pointed slightly downwards by the way. Not as extreme as Su-27 so they are only visible in 3-view drawings but still..
From more advanced perspective, it gets even more complicated.. Su-27 uses airfoil sections throughout its body; its the only true blended wing + lifting body design ever created (Though MiG-29 looks very similar it isn't aerodynamically, its more comperable to F-16 than Su-27). A cross section of Su-27 from right next to its nacelles is still an airfoil. A cross section from its nose to the tail stinger is still an airfoil if you exclude the cockpit canopy. On a such complex design no one could say for certain how flow behaves for certain.. Su-27 is a true marvel of aerodynamics engineering, and since newer designs has to take other considerations like stealth shaping and have a fat fuselage with internal bay, it may very well be the peak of what aerodynamics engineering will ever create; as far as maneuverability is concerned that is.