Question

Why does lucario have 2 egg groups?

Answer 1

Lucario's two Egg Groups are Field and Human-Like.

...the Field group contains lots of land-based mammals...

Source

Lucario is partially based off a wolf, so it fits that description. As for Human-Like:

The Pokémon in this group are bipedal, like humans.

Source

Lucario is bipedal, so it stands to reason why it would be in this group.

Hope I helped!

Comments

Sniped.

Answer 2

I mean, why not give Lucario 2 Egg Groups? Plenty of Pokemon are like that. However, I can try to explain why those 2 Egg Groups were chosen. Human-Like is pretty obvious. Lucario is bipedal and looks like a humanoid. Easy, simple, done. Now, Field is a little bit harder to explain. Field Pokemon usually have larger movepools and are usually Mammals. Lucario has always had a large movepool which allowed to run both sides of the spectrum. Lucario fits this characteristic and also fits the other one, as it is based off of the Anubis, a man with the head of the Jackal.
Source: https://bulbapedia.bulbagarden.net/wiki/Field_(Egg_Group)
Source:https://pokemondb.net/pokebase/146577/what-animal-is-riolu-lucario-or-is-it-not-an-animal

Answer 3

Not only lucario but a lot of other Pokemon are simultaneously in two egg groups, take a look on charizard for exemple, it is on the monster and dragon group. That also makes chain breeding possible.