Time and circumstances.
They thought they had this race won with the already-recieved fraud mail-in votes and especially with the Dominion vote switching.
When Trump started winning decisively ANYWAYS, they had to stop counting and fill out 100,000 ballots in 6 states.
Utterly no time to do anything but mark BIDEN.
They had been perfecting that come from behind with found ballots so long it has become a cliche, a standing joke, "oh look, more ballots over here!"
It just blew them away, completely unexpected, the sheer numbers voting for Trump.