If Chrysalis had done nothing after the rest of the group left Twilight, her plan would have gone without a hitch. Why she reveled herself, brought Twilight to Cadence, left them unsupervised with only a few brainwashed bridemaids to guard them is anyone guess. We know she wanted to sadistically gloat to Twilight…the only logical answer for why she didn’t just seize Twilight’s and Cadence Mind for that matter as that she just couldn’t…Cadence being an Alicorn may make her immune to Changeling control…but Twilight,being emotionally devastated and fragile should have been easy prey despite her normal intelligence. I guess the only reason is that chrysalis was stupidly arrogant and just wanted to be a sadist so she inadvertently went out of her way to put Twilight on the track to ruin her plans…just to gloat to her for a few seconds.

Maybe for the same reason why she didn’t mind control half of Canterlot at the same time.

(Later that day…) Chrysalis: “So apparently, for all her organizational skills, Sparkle is terrible at planning social events like weddings. I still don’t know how she managed to bungle things up so badly that the entire city ended up at the base of the mountain. On top of that, her dancing, such as it is, confused my army so much that they ended up attacking an unoccupied desert two hundred leagues away! This day couldn’t possibly get any worse…”

Maybe she can only brainwash one pony at a time? Or maybe Twilight is too smart?  
Just spitballing here. :P