On the Dissapator, the gas block is 8.5" from the muzzle, exactly where it should be to get the correct cycle and pressure curve for a 5.56/.223. The gas port location can be varied, but, the gas port size must also be compensated to make up the distance from muzzle. 8.5" from the muzzle is the perfect combo of pressure and time to operate the bolt/carrier. The Dissapator has a gas block under the handguards, and a Front Sight Base on the muzzle without a gas port/tube. You could move the gas port to the FSB, but the port would have to be huge, therefore pounding the heck out of the carrier/bolt to operate the weapon. If you ever shoot one of the pistol variations of an AR you will know what I mean, it is one hell of a kick to operate the short barrel/large gas port.
Now if you mean "why does'nt Bushy make an 18" variant" that is a different question, I believe that would be a great idea.