How does she even end upp getting kidnapped in that scenario??? She has guards around her and the prince is looking at her?
Well the meta reason (for the plot) that somebody else mentioned works, lol. If I had to give in-story reasons:
A.) Obviously they didn't take enough guards. Asher is proud of how peaceful things are, and the extremists had just been dealt with, so it seems like they were lured into a false sense of security. Also probably wanted to have something more like a normal date (just blending into the crowd) as opposed to something like a royal procession.
B.) They were attacked by a large group, so presumably the guards were largely busy. On page 17 you can see that Asher also has a sword out and it's crossed with somebody else's, so even though he sees her being taken, he's not free to chase after her
C.) It seems like the assailants mostly came from one direction, so she wasn't expecting one of them to be waiting in an alley in the opposite direction, and she doesn't have a reason to suspect she's the main target of the attack
D.) One of the first things they teach in first aid classes (or at least the one I was in, lol) is to assess the situation, because you only make the situation worse if you rush in, get hurt, and now there are two people to treat. I only bring it up because I think it speaks to the fact that, at least for some people, if they see an injured person and they think they can help them, their first instinct is going to be to rush over to help them. It was dumb to leave the immediate area of protection, but it was a hectic situation and she was just trying to help a person 'in need'.
But ultimately, yeah, plot, lol. Heroine making a space for herself right quick, and is an entrance worth making if it's not dramatic?