All top-level domains work just fine anywhere (with exceptions that aren't worth discussing for people in countries that run their own root servers). If you want ".tv" to stand for "television" instead of "Tuvalu", that's fine.
The point is simply one of perception. If I sell doorbells and my business is called "Ding Dong Bells", then I can use DingDongBells.org, just fine.
The problem is that I can spend zillions of dollars advertising my business in magazines and on TV as "DingDongBells.org". No matter what I do, a significant proportion of internet users in general, and this is very true in the United States, are going to see my advertisement, and then dutifully type in DingDongBells.com. It is simply a fact of life that people expect internet domain names to end in .com.
.Org was originally established, along with .com and .net, by Jon Postel who had sole authority over the root at the time. It was simply a decision by one person who thought that it would be a good idea to separate "commercial", "network infrastructure" and "other organizations" into different top-level domains. Today, PIR, which runs the .org registry, promotes and markets the TLD's to non-profit organizations, which desire to distinguish themselves from "commercial" organizations. Marketing aside, anyone can register and use a domain name in .org for whatever purpose they would like.
Cowboy boots are boots made for cowboys. Can you buy and wear cowboy boots even if you are not a cowboy? Surely you can. Can you wear cowboy boots and play tennis? Yes, if that's what you want to do. The point is that while you certainly can play tennis in cowboy boots, and nobody is going to stop you, most people prefer to wear shoes intended for playing tennis.