It's because most mages are particularly really strong early game and with the amount of money they can get from spell thief/ ancient coin they can buy decent items. Since ADC's are relatively weak at early game these mages become the dominant force at bot lane. Even the traditional supp cannot shield/heal the burst of these mages. These mages were suppose to be mid or maybe top only but with everything being cheaper now, they are viable in the support role as well. If gold was decreased from ancient coin/spell thief then the traditional supp will get hit very hard. If these mages get nerfed or their core items get nerfed then they wouldn't be able to play mid.
Maybe if the traditional supp received buffs or they made their items even more cost efficient, than they already are, maybe then you'll see more traditional supp being chosen to counter these mages bot lane. As well as, traditional supp spoon feed the adc much more so stronger and healthier adc will dominate the enemy team mid- late game.