Apparently by not filing an injunction, it delays the case significantly, Microsoft can still close but FTC can drag this out for a long time, if they filed an injunction, they'd have to do a quicker court case.
If that is true, then I'm even more convinced they know that they have no case, they're purely relying on either CMA or EC blocking the deal and if either one does that then FTC can say "we beat big tech!" because Microsoft will abandon the deal. If EC/CMA approve it with concessions then FTC will, knowing they have no chance in court, instead get Microsoft to sign the same concessions with FTC, again, making FTC look like they gained a win.