When the powers that be decide to block a merger, they have to make the case. There's looking at the proposed merger and being able to say "this is bad and may be antitrust," and then there's actually
making the case when it comes down to it. That takes time. And you can expect companies like Adobe to fight rather than just saying "oh, you don't like this? OK. We'll withdraw."
The DOJ and other bodies voiced opposition early on. They signaled quickly that they were not in favor of this. The interim between those actions and today were the regulatory bodies actively making cases against it and/or negotiating with Adobe & Figma how they could proceed in a way that the regulatory bodies would want to approve of the deal.
The whole idea that this was "half-assed" in some way is misguided.
Note that I worked for Red Hat while IBM was acquiring it. It took from (IIRC) October 2018 to July 2019 without strong opposition. When you're talking about companies that have that much impact on a market, it takes a while.
And it should -- the larger market doesn't benefit from waking up to find out that a major software supplier has been gobbled up overnight. You can see the pain that VMware employees + customers are going through right now and they've had a lot of time to process the idea.