It's not really a surprising result, a lot of this is due to recognition and William & Catherine have been much more visible in the past couple of years, esp since the Queen's death.
They/He always was popular, but he was slightly overshadowed by Harry and his media attention for a few years, esp in the US.
I expect his popularity levels with fluctuate slightly but generally remain high. He's now next in line, and likely to be King in a few years. He is young & handsome, his wife is beautiful and his kids very appealing.
He ticks all the boxes really.
But yes, I can see it getting downplayed.