The empire needed stronger central authority, and needed soldiers loyal to it. Severus created a more autocratic system, although in the end if it hadn’t been him it would have been someone else. The Roman Empire was always meant to become a military autocracy, since it had no deep cultural ties to bind it, nor any fixed loyalty to a single dynasty, that boat sailed long ago with Nero.
The empire ideally should not have seen major changes away from the successful model operated by emperors Domitian to Aurelius. The idea that emperors were above being merely chosen by soldiers was important for long run political stability, the fiction of the principate was important to maintain in the eyes of soldiers and equestrians. Severus undermined that fiction and emperors gradually became seen as merely chosen by soldiers and who could be replaced, cajoled into giving higher pay, the whole institutional framework of the system was exposed as being mere sand, and it came crashing down when his successors proved less competent.