I mean if you want to get technical, KitKat (4.4 in 2013) introduced verified boot. So from Android 1 to 4.4, it took about 5 years. I believe some form of sandboxing has always existed in Android, but the earliest version I can find online was in Android 5.
I feel like with the backing of Google, they were able to implement such tight security in their mobile OS without much pushback. Mobile Linux in it current state is entirely hobbyists with very few daily drivers. Unless someone can release some stunning Linux mobile hardware that a lot more enthusiasts buy I don't think we will see any sort of major progression in mobile Linux for some time, as the current method most mobile Linux uses is replacing the bootloader on the phone with an open source implementation which takes a lot of man power to achieve, and it would take even more to make it secure.
I would absolutely love to be proven wrong about the time frame however. The sooner secure Linux phones hit the market, the better the world will be.
I'm not really sure what about the sandboxing makes it difficult to use. Most of the permissions are switches you can just toggle on or off. You can also root phones that run custom Roms (which are the only phones that are worth a damn IMHO) if you really want access to the entire fs.
You're more than welcome to use a less secure system, but most people would generally prefer a secure one.