The main reason Riot is forcing 5 Positional Ranks for 2019.
Knowing that "Runes Reforged" failed for both competitive and casual players and that people are leaving League, Riot's next plan is to create a battle-royale gametype.
"Nexus Blitz" is Riot's desperate attempt to create a game-mode aimed for casual players for its fast paced, RNG gameplay.
If Nexus Blitz becomes a permanent game mode, the population will split causing Auto-filled games to be more prevalent in Ranked/Normal SR games. So, Riot is creating 5 Positional Ranks to hold players hands when they get frequently auto-filled.
tries best to look inconspicuous and begins nervously whistling