I don't think it was a morals issue. The rumours were that Musk wanted to turn Twitter into an (even more) unmoderated hate speech cesspool and drive off most of the (wealthy) userbase. Under that possibility advertisers were wise to wait and see instead of buying slots early (even if then they lost the usual discount).

That didn't happen, and Twitter is now about as terrible as it's ever been, so they're coming back.