Luke and Selina had discussed grabbing the Stones many times before this; the idea had come from Tony's attempt to steal the Stones in 2023.
However, they had never fixed on a certain person to grab the Stones. Instead, Knight, Big Dipper and Selina would play it by ear and take the opportunity if they saw it.
Selina hadn't awakened her Evolution ability during the fight in the warship. There were a few times when she wasn't far from the glove, but the chance of failure had been high.
Given Thanos's combat awareness, if she couldn't succeed in one hit, it would be very hard for her to make a second attempt.
Right now, Selina had the Evolution ability, and was immune to most of the Stones' energy. Thanos even planned to take Luke down with him; even if she didn't make a grab for the Stones now, it would be too late.
Also, Luke was confident that it wouldn't be Selina whom Thanos chose to take down with him.
