Because it wasn't a State Visit - it was an official visit. And the protocols for these are different. The Queen always meets a visiting Head of State even if the visit isn't a state visit, but other members of the Royal Family generally do not.
There was nothing unusual in them not being there.
A formal State Visit will be different. The Prince of Wales (and his wife) and the Duke of Cambridge (and his wife) would be expected to be in attendance at the State Dinner, alongside the Queen. At least two or three other senior members of the family would also be in attendance, the idea being to ensure there are enough royals there that all the VIPs get a reasonable to interact with at least one of them, but obviously people like the Duke of York, or the Duke of Kent, or the Earl of Wessex will be spending more time with other guests beside the Guest of Honour (in this case, President Trump).
(I used to be a protocol officer for Her Majesty's Australian Government, and had to interact a lot with similar people working for Her Majesty's (Britannic) Government. Understanding this stuff was my bread and butter).
it is always cool to learn who our fellow freepers are...gives us a chance to look into another world. Thanks for posting :)
aha. now that leaves me wondering why i did not run into that information in my prior reading of articles about that particular event. some articles or comments led me to believe that some of the royals might have eschewed attendance in a deliberate snub of trump. it sounds now with your “new” information that that prior information may have been erroneous. thank you very much for the clarifications and corrections!