Part of Wealthfront mission was to encourage younger people to save. They had a nice "savings tool" that you could play around with. Similarly how Robinhood got lot of people in to stock investing because the app was engaging and easy to use (for good or bad).
Fidely and Vanguard apps and websites are quite horrible to navigate and there are many more hoops to go through just to open an account and then you need to figure which funds to pick and how balance etc. With Wealthfront you just download the app on your phone, add some info, your bank account and set like that you want to invest like $250 twice a month and then forget about it.