Manufacturing wouldn't be any more difficult than forging a spear in the long term, but there's a couple of reasons why tridents are an inferior weapon to spears:

• While they may have more "poke," that poke is much less powerful, and isn't able to pierce armor like a standard spear can. One thicker point > three thinner points.

• The primary purpose of the spear on the battlefield was to extend reach and to combat cavalry. A thicker, singular blade was much more likely to kill or significantly maim a horse than a thinner trident prong, because it has more penetrating power and has less resistance when piercing the horses hide, and the pronged nature of the trident would stop the blades past a certain distance, giving it a lower chance to seriously wound (While many spears had a cross guard or the like to do exactly that, they were usually farther down the pole than the blade itself).

• A trident was a clunkier weapon to wield than a spear, usually having a weighted front tip. While not serious enough to change the way you'd hold and wield it compared to a spear, when combined with the other drawbacks, it's the third strike against it.

Ultimately, a trident is a fishing tool, designed to snare smaller fish with many points to prevent them from slipping away. It isn't really designed as a combat weapon, and even the Roman gladiators who wielded them were a mock-up of a fisherman (Their weapons were the net and trident, where the net would entangle and leave their fleshy parts exposed to the barbed trident), and weren't designed to be useful in a battlefield role.