I think they are popular because the hardware is (or was) good and it is (or was) easy to install LineageOS on it. I think I never used an Android device without changing the OS from day one, and Samsung was great in terms of quality and battery life and made it easy to root and change the OS. Once I bought a cheap Xiaomi phone of seemingly good quality. I had to wait 2-3 weeks before installing LineageOS on it, worst experience ever (plus I had to use Windows for that). The RAM was good, battery life also, but the screen on Samsung devices are like on Apple devices, whereas screens of Xiaomi were bad. I have read Sony also makes it easy to install OSes on their phones, plus they have a headphone jack.