Just looking at the subject title, one reaction would be, "Why not?" Another reaction would be, "Because it is a choice".
For me, those could be some reactions but there is much more to it than that. I am old enough that I had been in the software business as a professional a couple of years when the IBM PC was first introduced in 1981 to the consumer and business. People were using PCs before that, but they were definitely mostly serious hobbyists, small businesses and those who could not afford mainframe or even minicomputer systems.
In 1982 I switched assignments and joined a small advanced development research team at General Motors to investigate the possibility of getting the corporate software engineers on PCs that could upload their programs from PC to mainframe. We also put minicomputers in the mix to look at as departmental servers and multiuser systems for small teams. As such, I looked at MS/DOS, the mock ups of Windows that various people were trying to put together, and UNIX systems on minicomputers. The PC felt more "personal", but the minicomputer definitely had more power, infinitely better tools, and more imminent promise. I chose to pursue the mini route and UNIX, and took over an advanced development project to use the UNIX system as a communications hub to multiple sources using multiple protocols, and also to develop interfaces between the PC and the mini to form an early three tier network.
When Linux came out, I did not pursue it right away, but when I started hearing about how it used GNU software for utilities, I already knew most of them - and was using them on UNIX workstations. I wondered if Linux could have the "feel" and "friendliness" of the PC and the flexibility of the mini and UNIX. I found at first the answer was, "well, it's not quite there, but it is close enough that I can taste it". That was my reaction in 1995 and the distro was Slackware. It interested me enough to purchase my first home PC, though I'd used these systems for years at work.
It grabbed me right away. It took several more years to ramp up my usage because I did not yet have broadband at home. Once I got broadband in 1999, that was phase two. I entered graduate school and wrote most of my papers on the future of Linux and why I felt it could be a great success.
Once done with school in 2001 I started using Linux every day and have ever since. I have about a dozen Linux distros actually installed on my home systems at any given time, and hundreds of CDs and DVDs dating back to the nineties from the systems I have either tested or used over the years.
I use it because it brings convergence of styles for me - enough usability to suit me without having to sacrifice flexibility; I take flexibility over simplicity, but I appreciate a reasonable measure of each, and for me, I've now had it for years. I do consider myself an experienced user; not everyone will have the experience or interest that I do, so I never try to tell others they must use Linux, but I do tell them how useful it has been to me and how many alternatives I can choose from - if alternatives scare them, then I know it is not for them, but if they like alternatives, I try to help them out. That is one of the reasons that I am here - to discuss distributions, try to assist the informal "help network" and provide my historical insights and background.