Because of the drawbacks being greater then the advantages. However, I don't know of all the clamp types. So if you see a uncommon clamp that you think will work, let me know! If it's a common type, I've probably already checked it out.
Before you can find a better solution, you have to find the problems with the current one. The issue with the worm-drive clamp is that the drive mechanism is large and protrudes from the side of the lower. That's it. So for any other solution to be better, it needs to be more streamlined, while also meeting all of the requirements that the worm-drive clamp meets.
Other clamp types don't meet both of these requirements. Common problems:
Not more streamlined.
Too thick / stiff to conform to the non-round section of the buffer tower.
Not as strong
Not to say that there are not better options to the hose clamp! There are, and I am working on it ;)