First of all, there was no order to "Stand Down." Becker News made that up.
The linked
Times article says that
and
And yes, it can be explained: They fucked up. Some or all of Sund (Capitol Police Chief), Irving (House SaA), and Steiger (Senate SaA) ignored intelligence warnings or underestimated the crowd, and some "top general" from the Army held the National Guard back because he didn't like the "visuals." (
source) All three of those named paid for it with their careers.
Then they got surprised by a well-coordinated direct attack. An attack which, it should be noted, was staged by coordinated, determined Trump supporters who believed they were taking their orders directly from him, and whose stated goal was to overturn our democratic elections.