jkhuggins

Many reasons.

Some folks decide that the physical pat-down that accompanies opting out is more invasive of one's privacy than the AIT scanners. Different people have different views on privacy; I can imagine a survivor of physical assault preferring an electronic scan rather than a physical pat-down. (More folks might feel this way now than in 2010, since the remote viewing of detailed body images has been eliminated from (almost?) all AIT scanners.)

Some folks might have opted out due to the radiation from the MMWbackscatter-based machines. Since (almost?) all of those machines have been removed from service, their reason for opting out may have been eliminated.

Some folks decide that their time is more valuable than their desire to avoid the AIT machines, and are willing to sacrifice one aspect of their privacy for a faster checkpoint experience.

Some folks now qualify for PreCheck, or for the exemptions granted to various (small) population groups, and therefore no longer have the opportunity to opt-out of AIT scanning.

Some folks don't know that they can opt-out, and so they don't exercise a choice that they don't know they have.

And some folks simply don't care about the issue.

Any assessment of what percentages of the traveling public fit into those categories is at best a guess --- a guess that probably reveals more about the speaker's opinions than the real reasons people aren't opting out.