I believe I saw once that 2/3 majority is needed in the ACC to approve a new member and internal politics dictate that any formal announcement of a new member requires a unanimous decision.
At the time, the ACC had 11 members (excluding Maryland) with 3 pending members (ND, Syracuse & Pitt) with the belief that the 3 pending members did not have a vote at the time. Right off the bat, Florida St, Clemson, Miami, and BC were against UConn. Plus, Syracuse would likely been whispering in people's ears that they were against UConn, too, with Pitt doing whatever Syracuse told them to do as they saved their arse.
So before the proposal to add UConn even reached the table, UConn likely 7 votes for and 4 votes against, which may or may not provide 2/3 majority depending how one rounds the numbers. Throw on the fact that 2 of the conference's new members would also be against UConn and the timing of the NCAA ban (frack u, Syracuse hypocrites), UConn's chance at the ACC was doomed before it even began.
Thus, Louisville, which is as much a basektball school as UConn, was able to market itself to the FSU 7 Clemson block and with UConn doomed, it became unanimous to add them instead.
Could have UConn done a better job marketing itself to the ACC? Yes. Would it have made a difference. No. If CT has not sued BC and Miami presidents in 2004, would it have helped? Maybe, but, Miami would side with Florida St and Clemson while BC (and Syracuse) would continue to be afraid of UConn's potential (and still is). The only way that UConn could have won out is if ND came along and said, UConn or we won't join; but, ND only looks after ND and no one else.