Its because the quality of most minigames on servers is poor, unless they pay somebody to make them, another reason is because players generally like pvming and pvp, and would rather do that than play a minigame, however i do have a suggestion for one.
Make a pvp based minigame, in which 2 players will duel, and they will be given a completely random gear setup, and must fight, they could then earn points from winning the fight which could go towards a store, which contain the custom items which you do not want to be earned through pvm/pvp.
(idk tbh i just thought of this quickly, have fun making this, if you do
)
Best of luck!