The main reason is probably that you can swap characters mid battle. There's simply too many arts available for most players to remember what each XC2 style icon means.
They broke down the icon into main function (atk, heal, field, debuff, etc), reaction icon and AOE icon. These are the basic and most important things you need to know.
While they could have extra icons to inform conditional effects, they probably thought it would take away some focus from the other icons.
Also, when you toggle fusion arts, the left sword icon merges into the right sword icon to indicate you trigger them with X, Y and B. So, if they have N main icons, they need to make NxN fusion icons, so even more reason to not go wild with them.
Personally, I'm warming up for this decision, my only problem is that I can't tell with a glance which reaction is it (on my 6.8" phone through YouTube), hopefully that won't be the case. One thing it could be nice to add is the exclamation from DE to inform when conditions are met, as it would help with classes/arts player is not proficient yet.