Why haven't they accepted one already as there's been several requests and all have been denied so far. I hope this isn't the case and one will be accepted eventually but I wouldn't put all my money on it.
Currently the main problem is that the majority of the market making happens on exchanges outside the legal reach of the SEC. In case of manipulation, they pretty much can't do anything to combat the bad actors.
If eventually US based volumes grow, and it's no longer unregulated exchanges dominating the whole industry, they'll be more likely to let one come through. From there I believe it's only a matter of time.
I also disagree with you in that I do think it will have a significant effect on bitcoin. We already saw a drop in value when a couple of them were denied before, and I think one finally being given the green light will be very exciting for people and cause increased demand and a price spike.
I don't rule out any initial speculative hype resulting in the price to increase. I was more referring to the actual demand for these ETFs as instrument, which in no shape or form is guaranteed to be of significance.