This was forced by a regulatory body. Strangely, it was forced after the transaction had closed rather than blocked before closing.

That's because Facebook didn't wait for the UK regulatory body to come to a decision before completing the acquisition.

You don't have to tell the UK Competition & Markets Authority about an acquisition before it happens, but if you don't then you risk something like this happening. While an investigation is ongoing it may block some steps towards integrating companies and it gets about six months to make a decision. Facebook then spent a few years appealing and negotiating. Presumably giphy spent all of this time in some sort of limbo.

For different reasons though.