SEC knows that approving a Bitcoin futures would not affect the market much and there are fewer supporters of it. While in Bitcoin Spot ETF there will be a big difference. That is also the reason why they are also open to the approval of Ethereum futures because they know that fact said above.
I don't think this is about the security of the investors anymore, in my opinion's an attack on Bitcoin to keep it in control because once they approve the Bitcoin Spot ETF, something big could happen. Just imagine it being traded on the same platform as gold, stocks, and bonds. That will certainly make a lot of difference for the Bitcoin market and SEC doesn't want that to happen, yet.
Cancellation shouldn't be a surprise really, as they have been cancelling that since 2013. And i frankly don't think that issue about security of the investors would be all about market manipulation. As Bitcoin is new kind of asset that requires new kinds of safety measures for holders. Ironically, the very essence what makes bitcoin valuable, also makes it a dangerous asset to hold for anyone not tech savvy enough. Laws on insurance and customer safety lacks behind whenever we get new disruptive technology. I mean laws on the internet itself are still pretty rough and developing.