I also think Princes Harry and William have a strong desire to control their own "brand." As the Princes grew up, if I remember, the Prince of Wales had staff, but less independence from BP in general. The strong structure we see today at Claridge House was in its infancy. The leaks, apparent infighting betwixt the general BP staff and Charles' followers as he married resulted in mixed messages and eventually, Charles moving his staff to CH.
I think all learned from the experience and it did not surprise me when the Princes and Catherine announced they had established their own office.
Harry and William - even as/if they continue full time in the military, in some ways need a support staff MORE than a full time royal. I would imagine scheduling for Prince William - around his odd military schedule has been very difficult this year. With Prince Harry on active duty, he has almost no time for the next four months - with a lot of business that just keeps needing attention. That's what trusted staff are for.