You don't have to speculate, because they gave you the reasons themselves:
"the Commission is disapproving this proposed rule change because it does not find the proposal to be consistent with Section 6(b)(5) of the Exchange Act, which requires, among other things, that the rules of a national securities exchange be designed to prevent fraudulent and manipulative acts and practices and to protect investors and the public interest."
Basically, they're concerned about Bitcoin's use in fraud and scams, which cannot be changed as it's part of Bitcoin's fundamental nature.
it cannot be changed,but the Commission opinion may and ,probably,will
we all know too well that all of the commisssions and councils are not independent
maybe the time is not ripe for it to be approved,but I'm sure that their "decision" is influenced by other parites involved
as soon as there is more legal freedom for bitcoin and more mass adoption,there will be no reason to reject it anymore
besides,there can be numerous attempts at securing ETF and I am certain brothers will try their best to get approved next time