Riven disposition is based off of the weapons usage among the player Base, they are not to make weapons more powerful, but to provide stat changes to the weapon to make it functionally different from its unrivened version. The idea being to increase the variety of weapons used instead of only the meta weapons.
A side affect of this is the player mentality to make everything as op as hell, so now many players want rivens for meta weapons to make them even more op. Unfortunately, this causes usage to increase, and so the disposition gets lowered.
Things like the gorgon and viper disposition got lowered because DE are now reviewing disposition every 3 months. Within these 3 months, baro has had prisma gorgon and wraith viper, so there would have been a spike in these weapons usage.