Regulatory incentives, perhaps. I may be wrong on this, but I have my doubts on how it will play out in the long term. I believe the more important aspect is the type of ETFs that will be approved, at least initially.

Part of what held up the ETFs was the SEC requiring applicants to change their ETFs from in-kind redemptions to cash creates. I hope I'm not being patronising by going over the differences, but in-kind redemptions would allow investors to have their Bitcoin returned to them when their shares in the ETF were sold. On the flip-side, I'm not sure why someone who self-custodies would want to hand their BTC over to an institution.

The SEC requiring cash creates ETFs means that investors hand over fiat in exchange for ETF shares, and when they sell, they get fiat back. So the people buying ETFs that operate on this basis will never actually own Bitcoin, allowing the supply to slowly be funnelled to the institutions for holding (one a 1:1 basis or otherwise). Cash creates ETFs will have more middlemen involved, which means more money to be made from fees, but easier access for first-time buyers.

Last I saw, Grayscale was the only one that hadn't amended their application, for if they do, they may have to sell their holdings (600K BTC) which has the potential to negatively affect the market.

I'm just not a fan of the entire thing. They may start off with a 1:1 backing, then change the structure of the ETFs once they hold enough of the asset and investors have gained some confidence in the scheme.