I thought the point was that the rules were changed so that all of William's children would be princes and princesses from birth. Before the change, only George would've been a prince, and Charlotte and Louis would've had to wait until Charles was King.
As has been explained above, the change was made because if Charlotte had been born first, she would have been next in line to the throne, but still styled as the 'Lady Charlotte', while her younger brother, whom she outranked, would have been 'Prince George'. That potentially awkward situation wouldn't apply to Harry's children, who are not in direct line.
The same change wasn't made for Harry's children (and I'm not saying I disagree with that, it probably was the right call), and then the lack of title (which was a choice) was linked to provision of security. Even though security should be provided according to need rather than title
Security is provided according to need rather than title.
The notion that being a prince automatically means round the clock police security is sheer nonsense. At least one of the queen's own children, Prince Edward, does not get full police protection.