the only reason Santana believes that Quinn is not coming is because she knows about how Quinn feels about Rachel, she was there through it all: she could see it while she drew pornographic pictures of her in the bathroom stalls, she could see it when she covered Rachel’s drawings with hearts, she could see it when Quinn covered Rachel’s pictures so she wouldn’t torture herself, she could see it when Quinn made speeches to Rachel, or gave her significant looks, or had to suppress a smile when Rachel started to ramble. She was sure because Quinn every once in a while would get terrified at the thought of Rachel marrying someone else and Santana and Brittany had to calm her down and try to convince her to talk to her. To try.
That’s why when Santana heard that Rachel was getting married today, after Regionals, her first thought goes to Quinn. That’s why when Quinn, wearing her cheerio uniform again, informs her two best friends that Rachel sang to Finn and only Finn, she hugged her with all her strength. That’s also why she was even more surprised that Quinn said that she would go to the wedding in the first place. Quinn informed her that she had no intentions to let it go now, because she knew she would never forgive herself if she did so. So she would make her first and maybe last real try to tell Rachel how she feels about her. Santana is surprised, yes, but happy for her. Brittany is even more excited.
So when Rachel tries over and over to reach Quinn and she’s not there, Santana simply thinks that Quinn’s not coming. She lost her courage. And she couldn’t be here, doing nothing to stop this. So Santana’s tone is not bitchy, is simply frustrated because she knew that maybe, just maybe, if Quinn tried, she could be a bridesmaid at another wedding. One that could take place in years, in New York maybe. One that her best friend could be a bride too.
So she tries to face it: Quinn was not coming.