I can think of at least two reasons why:
1) The CSAM debate is still not quite over;
2) This is the first time Apple presented iOS 15 as a “quiet” and “by the way” update, somewhat hidden at the bottom of the screen. If a user already had iOS 14.8 installed, they would not even get the usual red update bubble in Settings. This time around people had to actively opt in to install iOS 15, which is very unusual.
1) The CSAM debate is still not quite over;
2) This is the first time Apple presented iOS 15 as a “quiet” and “by the way” update, somewhat hidden at the bottom of the screen. If a user already had iOS 14.8 installed, they would not even get the usual red update bubble in Settings. This time around people had to actively opt in to install iOS 15, which is very unusual.