I used to think that it was because they were waiting to do a big “charity powerhouse” pr drive for Meghan this month, but that does not seem to be the case. I can only assume that it’s because the numbers are pretty low.
An anon thought that maybe they can’t release the numbers because they did individual charities instead of a big fund like Will and Kate, but I don’t think that’s the reason. They can ask the charities if they had a bump in donations between the announcement and the wedding date and publish that. The charities will know exactly what they got and where it came from.