There are several reasons.
1. The source code for almost all software in Ubuntu, and most other Linux distributions, is under a free software license. The idea behind free software is that anyone should be able to modify and view the source code free of charge, without signing non-disclosure agreements that limit your rights.
2. If people had to pay $120 for ubuntu, fewer would be willing to try it. Offering it free will make more people willing to try it. Ubuntu's parent company, Canonical LTD., can still make money through large-scale support agreeements, such as with server OEMs and computer OEMs like Dell.
3. Mark Shuttleworth is a nice guy. He wants you to experience #1 as much as possible. To remove even the extremely low cost of downloading an ISO image and burning a CD, he will even mail you one free. This also helps push Ubuntu into developing markets.
ubuntu has been very effective at making it user friendly, it's free and free stuff will never go out of style. Also Canonical has a very good marketing team
Something concerning to Microsoft. Linus wanted to prove that He could build an Operating System capable to face Windows, for free.
Linus Torvalds released it for free. It wasn't particularly useful when he first released it, so nobody would have paid money for it, anyway.
Ubuntu is a free system program. Therefore, all of the software which comes from Ubuntu is free as well. One might want to check out from the main website of Ubuntu.
Ubuntu is a free, supported debian/linux implementation of Unix. As such, you can do nearly anything that you can do in Unix in Ubuntu.
Ubuntu is free, it can not be pirated.
It is free software, and will not cost you a penny.
ubuntu is free to download
Its completely free.
Ubuntu Server Edition is a free and open-source operating system for use in servers.
UBUNTU
PowerISO is not supported on Ubuntu. There are a multitude of programs that can do the same thing as well as or better than PowerISO, available for free in Ubuntu's repository.
Ubuntu server is a free download service. This is a very reliable program and has helped thousands to stabilize their computer programming. This would be a great asset.
Ubuntu is the name for the popular Linux operating system. It is free to download and use and popular with program developers. It can easily be found by going to the Ubuntu site itself.
Windows 10 is currently a free operating system and can be downloaded on the Microsoft website, Ubuntu OS is also a free operating system, it can be Downloaded from the Ubuntu website Ubuntu.com/download