Why isn’t it played as often any more?
A few reasons:
When combined with stacking Celeste Claws, the spear doesn't really offer a lot of additional enmity. It's, essentially, diluted and doesn't make as great of a notable impact on damage to make its Ougi worth the trade-off. In fights like FaaHard, it was pretty powerful since you couldn't stack Claws without some kind of drawback, so Advocatus's boost got to be more pronounced, along with its Ougi being fairly useful. Outside of that one fight though, nothing else really needs it since other endgame fights need other classes.
Xeno weapons, traditionally, transition into your Astral weapons. Astrals are also EX-based and have the MH requirement (until ULB), but are stronger than Xenos since they use Unworldly instead of Massive and provide some form of cap up. Even then, Dark has the potential to not even use their Astral weapon if they're using two Pain and Sufferings since they have the potential to give A LOT of EX atk.
When combined with stacking Celeste Claws, the spear doesn't really offer a lot of additional enmity. It's, essentially, diluted and doesn't make as great of a notable impact on damage to make its Ougi worth the trade-off. In fights like FaaHard, it was pretty powerful since you couldn't stack Claws without some kind of drawback, so Advocatus's boost got to be more pronounced along with its Ougi being fairly useful. Outside of that one fight though, nothing else really needs it since other endgame fights need other classes.