The main reason is Google. .com and .org still rank better than less established alternatives.

The standard example for an alternative domain is .info. A website with a .info domain is harder to get to the first page of Google. The reason here is that .info domains are cheaper and can be registered anonymously. That's why many spammers and hackers use them, and that's why a .info domain is a negative ranking factor.

If you want to appear at the top of the search results at Google for some keyword, e.g. your brand name, it's a huge plus to use "yourbrandand.com".