Energy for "free"
Hi dtb
Main reason is that the antimatter is there for free and ready to burn, but you have to make bombs, they're finicky and otherwise banned by treaty. Andrews Aerospace has a good sub-critical design which uses a Z-pinch to magnetically compress fissiles to critical-mass that they wouldn't otherwise achieve - Mini-MagOrion, if you want to check it out. The pulse units aren't bombs because they can't explode except in a Z-pinch.
Adam