Multi monitor gaming as mentioned is expensive in display cost and the GPU it takes to drive that many pixels. Also, now a days there are some excellent Ultrawide curved OLED displays that have kind of taken over that space, like the Samsung Odyssey. Aside from that, less work for developers, as it's easier to support, and also less setup hassle for end users. A sign that multi monitor gaming is fading away though is the oft times you see fixes needed for multi monitor setups still to this day, a sign that not many devs want to support it anymore.
That said, the one tradeoff of ultrawide curved vs triples is you get less peripheral view. With Ultrawide if you're playing a car racing game for instance, the peripheral stops at your side mirror, whereas with triples it extends a ways behind it.