I agree that Lucario has well balanced stats. The thing is, the sets that are run on Lucario are mostly either Physical or Special (sometimes both).
When they are either Physical (Swords Dance + Extreme Speed + Close Combat) or when they are Special (Nasty Plot + Vacuum Wave + Aura Sphere + Flash Cannon / Dragon Pulse/ Dark Pulse /etc), then having the other stat higher makes no sense. The extra power could go in boosting said moves as opposed to reducing another attacking stat that is not used in battling.
Note that this is in reference to offensive stats and speed, whereas most Natures that are bad for the defenses are not not encouraged, unless the Pokemon has stats so low that a drop in it hardly matters (Like Kartana in Special Defense)
In case of mixed sweepers, generally, such a weaker defense is sacrificed for more power/speed. This is again based on the set, of course.
All in all, while Lucario has balanced stats, it is important that the primary role for Lucario is that of an offensive Pokemon, meaning it KOs opponent Pokemon. It is not generally used to takes hits, rather deal them out before the foe can move. Hence, here a nature benefiting Attack/ Special Attack / Speed is encouraged.
That said, Serious or any other neutral nature is not bad, specially in-game, and there are worse natures out there for it, like those reducing Speed for defense boosts.