The best explanation I've seen:
First, Dark type is actually the Evil type in Japanese, explaining moves like thief and foul play, just saying.
Criminals (Dark) often operate at night, which makes them more vulnerable to bugs. Also, while criminals are often alone, bugs comes in large groups.
Still sounds like a stretch to me
This theory seemed reasonable:
"The Dark type is an embodiment of dirty tactics. The original Japanese translation is 'Evil' type, and this is shown as giving the Pokemon of this type underhanded or sneaky ways of fighting. Examples of this are shown in various Dark type moves:
Pursuit : Dealing more damage on an enemy trying to retreat
Sucker Punch : Attacking before the enemy is ready
Fake Tears: Pretending to cry to put an enemy off guard
Beat up: Attacking with the whole team in an unfair 6v1 fight
Culturally in Japan, the villainous dark character must be beaten by the virtuous hero character. That is why Fighting types are super effective. They embody this heroism trait.
(Cont)
So why also Bugs? This is probably a reference to the series Kamen Rider, a show that has existed continuously from the 1970s to the present day and is a cultural phenomenon large enough that every child in Japan would know it. In this series, the usual premise is a Masked Hero riding a motorcycle fights the forces of evil with the Main character always using an Insect themed costume.
In fact, the Bug type move Signal Beam(シグナルビーム) is pronounced as English in the original Japanese versions, reminiscent of many Kamen Rider 'special moves'.
This insect association with heroism actually goes back further, with stag beetles representing the honourable samurai and becoming the basis for many real samurai helmet designs.
Another hint towards this connection exists before the Dark type was ever introduced. In generation 1, the villains often used Poison types. This is the only Generation where Bug types were super effective against poison!"
That’s my whole justification of this typing arrangement.
First, Dark type is actually the Evil type in Japanese, explaining moves like thief and foul play, just saying.
Criminals (Dark) often operate at night, which makes them more vulnerable to bugs. Also, while criminals are often alone, bugs comes in large groups.
Still sounds like a stretch to me
"The Dark type is an embodiment of dirty tactics. The original Japanese translation is 'Evil' type, and this is shown as giving the Pokemon of this type underhanded or sneaky ways of fighting. Examples of this are shown in various Dark type moves:
Pursuit : Dealing more damage on an enemy trying to retreat
Sucker Punch : Attacking before the enemy is ready
Fake Tears: Pretending to cry to put an enemy off guard
Beat up: Attacking with the whole team in an unfair 6v1 fight
Culturally in Japan, the villainous dark character must be beaten by the virtuous hero character. That is why Fighting types are super effective. They embody this heroism trait.
(Cont)
In fact, the Bug type move Signal Beam(シグナルビーム) is pronounced as English in the original Japanese versions, reminiscent of many Kamen Rider 'special moves'.
This insect association with heroism actually goes back further, with stag beetles representing the honourable samurai and becoming the basis for many real samurai helmet designs.
Another hint towards this connection exists before the Dark type was ever introduced. In generation 1, the villains often used Poison types. This is the only Generation where Bug types were super effective against poison!"
'
Also, likely just to help keep dark and bug types balanced and not over/under powered