system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Trek DS9] Why does the Defiant exist in the alternate universe? And why isn't the "teleporting bullet gun prototype" not standard issue?

The Federation does not exist in the alternate Universe - so why does it get a chance to shoot Worf's Alliance cruiser? 

When the Vulcan modifies the prototype projectile rifle with a teleporter, why does that not become the instant standard issue? Or why isn't there a team of people (perhaps like Navy Seals) who specialize in it? No one could deal with this gun. 

Although - I don't think that bullets would hold their velocity after being teleported either. If it needs to turn the bullet to energy to send it to a new place, then shouldn't phasers be able to "teleport fire" too? 

They created a super weapon and it is never used again.
gpt
The defiant exists because Smiley stole the blueprints when he went into the regular universe so that he would have badass ship to fight Worf with.  Smiley even brought Sisko back to the mirror universe to finish up building it.

As for the rifle, it uses a mini transporter.  In a war zone or other high security area, they have dampening fields that make the transporter completely useless.  Also the gun will not shoot through shields, which makes it uesless compared to a phaser.