I think there are three reasons. One is the cultural habit of a large dinner, the second is dinner precedes 12-14 hours without eating, and lastly, many people who start Bright Line Eating have their hardest time in the evening, as they used to do a lot of binge eating or constant grazing between dinner and bedtime.
The composition of your weight loss food plan is essentially what was passed along to me well over a decade ago, and I’ve seen it work for thousands of people since then. So it’s folk tradition more than science.
I personally am in favor of having the biggest meal at breakfast, the second biggest meal at lunch, and the smallest meal at dinner. That’s the way I recommend introducing foods on the way to maintenance. I like to bulk up people’s breakfast and lunch before adding anything to dinner.
Moving some of that dinner food to lunch (for example, the vegetables or salad) is fine. Just decide what your new plan is and then stick to it. Having more food at lunch will likely serve you well.