Question

Why do slakoth and slaking get such a bad abillity?

Comments

I guess so people know the nature of these two Pokemon (laziness). Slaking has it because without it, it would probably be too strong.

Answer 1

Gen 3 was the generation that introduced Abilities, and Game Freak probably wanted to show that abilities aren't always good, so they made Slaking, a Pokémon with ridiculously overpowered stats and then balanced it with an absolutely horrible ability. Slakoth also gets Truant most likely just because it fits thematically.

Comments

To add to this, the ability Truant was also likely created in conjunction with the Move, Skill Swap, in order to encourage Ability play.  The idea that Abilities themselves can be worked with or even weaponized, instead of effectively being additional "stat sticks".  There are a lot of interesting things that can be done by manipulating or moving around Abilities.

Answer 2

The ability Truant is used to nerf Slaking's great stats and attacking moves. Without, Slaking would be very powerful.

Comments

look at slaking's stats that is almost box art legendary stats it would be made uber
150 hp + 100 defense that is a very bulky pokemon
160 attack + 100 speed means it will outspeed and destroy a lot of pokemon
slaking also has good recovery with slack-off
has a decent 95 sp. attack incase you try to burn it
and worst of all slaking is a pure normal type meaning it is only weak to fighting type moves at 2x and because of slakings hp and defense will be able to eat up any physical move you throw at it and in gen 3 fighting was physical
meaning you would have to target it with neutral ice, water, fire, electric and special type buy slaking has a huge move pool so will be able to have an answer to almost any pokemon
even if they gave it an abilty like pickup which has no effect in battle it would be too strong