The military don't like antimatter weapons because they are fail-dangerous. They require active stabilisation.

If a nuke fails, nothing happens. If antimatter containment fails, your entire arsenal of antimatter weapons go up at once.

For a similar reason, nitroglycerin is strangely unpopular for both civilian and military purposes.

Interesting, so you are suggesting some sort of gun that creates antimatter in a remote location. That's a great idea!

A weapon that could create antimatter at a remote location would be a particle accelerator. One that could create non-trivial amounts of antimatter would cause far more damage from its direct consequences than from antimatter. By factors of millions or billions.

It would be heinously expensive and would require the kind of energy input that only gigawatt-grade nuclear power plants could provide. Rather than using a complicated, failure prone and inefficient way to transform nuclear fission into destruction, it would be simpler and more effective to lob a nuke with the same amount of uranium or plutonium.

Hence, for your day-to-day megadeath needs, thermonuclear weapons will remain the tool of choice for the foreseeable future.

Unless we could convince the enemy to use that weapon!

I guess thermonuclear weapons are cheaper, though...

To build either nukes or particle accelerators you need physicists, who are the kind of non-team-players likely to point out that nukes would work better.

So .. you don't read much sci-fi, do you?