I notice that every Pokemon type has their lengendary except Bug and Poison type. Why? are they too weak or some reason?
That is becuase they just didn't and until B/W there wasn't a fighting type
Well, now there are a few more.
Arecus can become Bug- and Poison-type, if it's holding the Insect Plate and Toxic Plate respectively. Buzzwole, Pheromosa, Nihilego, Poipole, and Naganadel are sub-legendary. Ultra Beasts are included in this category (check my source).
These are officially Legendary Pokémon in the code, but are sometimes categorised separately in other material.
Source
well actually there is both.
genesect is a part bug type
arceus can be any type with the right plate, including bug and poison