I would be interested to know how you know this. I, for one, have not booked a Saga cruise online since I became aware of the pre-registration system. Given that it seems to take Saga at least a month to deal with everyone who has pre-registered, it seems to me that many of us book our Saga cruises by phone.
I am quite happy to book and pay for things online, and will do with my Saga excursions, but I know a number of people who are reluctant to input financial information online and, given the number of scammers out there now, I can’t say I blame them. So there may be many Saganauts who use the internet for information but do they actually book and pay for things online?