Devs just never cared to further develop the game outside the online PVP aspect.
We never got any meaningful single player or story content. Just some hastily slapped together "world" with "lore" and not really expanding on it.
They also never cared to create a big community and keep it alive and engaging. They barely held events on Twitch or whatever, for example.

All they really cared about was skin skin skin, new character, skin skin skin, new map, skin skin, new character.
And well, shit just got stale. Game started getting too crowded. New characters just coming with awful designs or overpowered kits.
It was just a great game that got the worst devs in history.