The problem with William, in addition to his low engagement numbers, is that he doesn't come across as especially passionate or dedicated to anything, and at nearly 40 has no major projects or causes strongly associated with his name.
Charles is clearly a man with many deeply held passionate interests - to pick just one his long-held passion for organic gardening and organic food long before it was popular, leading to the behemoth that is the Duchy Organics product brand.
Philip had his Duke of Edinburgh scheme which he was clearly dedicated to.
Harry, whatever you may think about him, is clearly extremely passionate about and dedicated to helping military veterans, and also has a deep love of Southern Africa.
Anne is obsessed with horses, a former Olympic athlete, and works hard all round.
Camilla has her campaign to support rape victims which isn't glamorous and gets little attention, so it's clearly a labour of love.
Even Kate (despite being pretty work-shy) clearly loves and cares about children, is interested in photography and gardening, and loves certain sports.
William... what are his interests and passions? Yes he has patronages he visits, but there's no one thing that defines him the way organics and architecture define Charles, Invictus and Sentebale define Harry, and children and being sporty define Kate. He occasionally talks about racism in football , or mental health, or announces some bigshot environmental award, but none of them have resulted in anything major and none of them come across as issues he's genuinely passionate about or committed to.