Likely due to the regulatory difficulty in then brokering fractional share purchases. If your "index fund" includes a few stocks that have large share prices (

Likely due to the regulatory difficulty in then brokering fractional share purchases. If your "index fund" includes a few stocks that have large share prices (e.g. TSLA, AMZN), then to buy a single "share" of your fund would require quite a bit of capital. I don't believe the retail level investors that Robinhood targets are quite at that level.