This is my theory on why Kanto Pokemon are so shit
In the 1st generation games, Kanto has just been in a war (whether it'd be with Johto, Unova, Kalos, ect), which obviously means it's been damaged by war. Evidence of this can be found with Lt Surge, who has obviously fought in the recent war. You all already know this, so this shit is pretty obvious
As a result, the Pokemon living in Kanto got weakened by the war, resulting in lower stats and entire types being wiped out of the region. Most likely by nuclear weapons, or possibly a constant barrage of extremely powerful Pokemon tearing up the entire region
This would also explain how some types (such as Poison) became so common and prominant (combined with the amount of pollution in Kanto). Eradication of many types results in other types being able to take over