Of course things like software quality, bad UX (e.g. still none of the hands-off/continuity features work) are a reason for me: why would I pay the „Apple tax“ if „it simply works“ is no longer true?
Another reason is the hardware, and that’s a complex one. On one hand Apple hardware is really good, e.g. the touchpads are the best I know. But on the other hand they do stupid things like soldering the SSD and RAM onto the board or gluing the battery. At least the SSD should not be soldered, as I use my hard disks heavily (due to big databases) it is likely that it breaks before the computer is broken.
Also software freedom is a reason. I like the ideals behind the GNU project and think this is the right way.
But my absolutely main reason is performance. Linux performs so much better… I have a script touching and inserting about 2 million rows, one at a time. My Linux finishes the job within two hours, while my macbook needs six(!!) hours to complete the task. The overall performance is so much better, and disk I/O is in its own league.