Dogecoin has its niche, and could be adopted by millions of users, so it is normal that it is among the largest cryptocurrencies and that's why it's been included in this Robinhood list

One whole heck of a lot of cryptocurrencies are now on Robinhood, including the following. Bitcoin, Ethereum, Ripple, Bitcoin Cash, Litecoin, Qtum, Ethereum Classic, Stellar, NEO, Zcash, Monero, Dash, Bitcoin Gold, Lisk, OmiseGO, and Dogecoin. As they say on Sesame Street: Which one doesn’t belong? It’s baffling to see Dogecoin amongst these other major coins – but who knows?

https://www.slashgear.com/stock-app-robinhood-ads-no-fee-bitcoin-trading-25517052/