I don't think there's evidence that there is a standard time to announce a pregnancy in the BRF, so the assertion that there is a usual time isn't factual. It seems to me that the couples announce depending on their circumstances. Zara announced around 3 months twice--and then she didn't. Catherine and William announced early all three times. Meghan and Harry announced around 4 months. Seems like quite a range, and I think speculation as to why is just that, speculation.
It’s quite obvious why. Kate didn’t have a choice. Zara was conventional until she had two miscarriages. Meghan was trying her hardest to hide it until after the wedding despite some pretty obvious signs.