Customisability, sustainability and freedom were the main reasons for me.
The absolute key thing is that I get to control the hardware and I get to make the decisions that affect how I experience it, and that I get these freedoms as intended use cases of the device, rather than as aftermarket, reverse-engineered hacks.
Smartphones are to some extent only useful as far as you rely upon them in your day-to-day life. When usefulness breeds reliance, you need reliability. But mainstream phones cannot be relied upon. They are controlled by the manufacturers, OS vendors and app vendors, not the users. These landlord-like entities can change the functionality and user interfaces on a whim. User customisation is difficult and unnecessarily limited. Users are corralled into brand experiences.
And as if that wasn’t enough, the whole operating system of these other phones can be abandoned and stop receiving security updates just a few years after purchase, usually with no realistic possibility to install a different one. That, to me, is not a useful device. That is a liability. The moment you make it useful to yourself by depending upon it, you become a slave to these third parties. I chose to avoid that and have therefore never owned such a device.
To my mind, a smartphone ought to be personal, like a room in your own home. Instead, it’s often more like a room in a chain hotel. You can rearrange the furniture a little, and bring your own possessions with you; maybe put up a couple of posters if you’re careful; but you can’t redecorate or start drilling holes in the wall to put up some shelves and the housekeeping staff keep coming in and moving your stuff.
For these reasons, the Librem 5 is the first smartphone that I have felt is worth the cost of purchase, and the first I’ll own.
A big part of that is that it will run a standard, Linux based operating system with a mainline kernel. It’s a sensible way to do convergence, and it provides a nice, well-known, well-understood and well-supported modular platform for application software, system software and customisations. @johan-bjareholt summed up some of my feelings about Android better than I ever could.