Probably because someone took "ask for forgiveness, not permission" a bit too far.
https://www.cnn.com/2018/11/02/tech/uber-self-driving-tests/...
To be fair, they're proposing significantly improved safety procedures. To also be fair, there are not many people (as opposed to corporations) who would be out on good behavior 6 months after being convicted of vehicular manslaughter.
Of course, maybe that didn't happen, but between the idea that RH would build and announce a new product without running past the proper regulatory authorities, and the idea that the president of SIPC might just be wrong ... well, the latter seems more plausible to me.
Simply put, Robinhood has far more at risk than the SIPC by launching without SIPC insurance. The banking industry is heavily regulated and Robinhood has been in the brokerage game for enough now to know that a major product launch like this in a tightly regulated area requires massive amounts of paperwork and approval. A mistake like this could collapse not only this new product but also their brokerage. It's far more likely that they have done their due diligence before announcing and launching a product that has that big of a risk.
Edit: We don't even have to wait! SMH...
https://www.google.com/amp/s/techcrunch.com/2018/12/14/robin...