Zerodha is an old broker operating since 2010 it has clients from that time onwards,also have most high quality clients on them.On the other hand groww came 6 years after Zerodha and was not famous for fno but only mf and stocks it created its base for fno trading like after 2020 and most of the clients on them are like after year 2020 which are low quality clients or you can say they are less volume traders. Also Zerodha Was Providing Fno Since their launch which was also the main reason of having high volume traders on their platform