Because their news blackout campaign was supposed to be saved for the post-election steal, that's why.
If Nov 4 was the first-time rollout of nationwide full bore tech censorship, no one would have believed that it was happening. We pedes would have pointed it out, sure, but Big Tech would call us conspiracy theorists and Nazis and QTards and all the rest, and the normies wouldn't believe a word we said.
But Twitter and Facebook got trigger happy and blew the whole deal. They played their winning card too early, and everyone saw them trying to disappear a major national news story, and to disappear the people who shared it. And by doing that they lost the element of surprise, and created a precedent for assuming their bad faith.
And now we've got the records, we've got the receipts, we've got an explicit preview of their tactics, and we mustn't let anyone forget it.
So hammer it into the normies' heads: BIG TECH JUST TRIED TO VAPORIZE THE TRUTH, AND THEY PLAN ON DOING IT AGAIN COME ELECTION DAY.
You assume that their campaign didn't work, and everybody saw it. I am not sure how many people heard of the Biden organised crime, but have the feeling that the coverup is quite successful, and it will work again when they try to cook the election results.
Notice how only Facebook and Twitter had this knee-jerk obvious censorship, both of which happen to still be run by their founder with a god complex. Fortunately, the left's arrogance and hubris is often the right's best weapon. If they were more subtle, for example they started with that concern-trolling landing page about "harmful" content instead of banning the link entirely, then it wouldn't even be a story.
It's important to keep in mind this could have been Silicon Valley's test run to prepare for the real thing, they will no doubt be employing these tactics after the election. They might even be more blatant, because at this point they can't afford to lose and be reigned in by a Trump FEC, FCC and FTC.