iirc, the reason the SEC hasn't authorized any ETF's and isn't likely to authorize any is stated in thier decision:

"First, the exchange must have surveillance-sharing agreements with significant markets for trading the underlying commodity or derivatives on that commodity. And second, those markets must be regulated"

"Based on the record before it, the Commission believes that the significant markets for bitcoin are unregulated. Therefore, as the Exchange has not entered into, and would currently be unable to enter into, the type of surveillance-sharing agreement that has been in place with respect to all previously approved commodity-trust ETPs—agreements that help address concerns about the potential for fraudulent or manipulative acts and practices in this market—the Commission does not find the proposed rule change to be consistent with the Exchange Act"

Allow me to translate that into english:
The existing market of cryptocurrency exchanges are too decentralized......and as such, surveillance-sharing agreements can't be obtained (or have not been obtained).

The SEC claims they want names, account numbers and trade data when they suspect manipulation. Whether that is their intent or not is another question.

If the market moves volume toward privacy oriented DEXs (decentralized exchanges) with no KYC/AML, a BTC ETF could never be approved by the SEC, imho.

Heres the comments from the March 2017 Winklevoss decision. Every ETF denial in 2018 has had similar language requiring surveillance-sharing agreements.