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.