>>54299858It helps that Bulbasuar is No.1 on the original dex and is a starter mon.
Poliwhirl was just somewhere in the middle in the dex and you couldn't get one until you evolve a Poliwag. Also Poliwhirl is overshadowed with it's other evos while Bulbasuar shares it's popularity with Ivy and Venusaur in harmony.
Poliwhirl as a mascot was always a strange choice to me, and it being favored by
>>54299853 was the only reason it got it's face on products.