I don't know if you have noticed, except for some large chain brands, and very few coffee shops use trapezoidal filter cups to produce pour-over coffee, and now almost all of them are conical and large-hole structure filter cups.
The world's first filter bowl is the trapezoidal filter bowl of German Melitta, which is an inverted ladder shape, wide at the top and narrow at the bottom. And because the filter paper used resembles the shape of a fan, it is also called a fan-shaped filter cup. Due to the popularity of espresso at the time, pour-over coffee has always been a very niche home coffee utensil.
It wasn't until 1959 that pour-over coffee became popular in Japan, and Japan's kalita "borrowed" this trapezoidal filter cup and improved it, changing from a single hole to a three-hole one. This trapezoidal bowl from Kalita has become a classic among trapezoidal bowls. 
The first conical bowl was invented by Kono in 1968, and the classic V60 bowl in the conical bowl was introduced around 2000. In this case, Qianjie uses the classic Kalita 101 in the trapezoidal bowl and the classic V60 in the conical bowl, and maybe you can understand why the conical bowl is now used, and the trapezoidal bowl is becoming less and less.
Front Street used washed Yirgacheffe and Jamaican Blue Mountain coffee for comparison, and the brewing parameters remained the same. The flavor of the brew is as follows:[Yirgacheffe].
V60: Subtle white floral aroma and citrus fruit aroma, citrus-like sweetness and green tea rhyme, clean taste and obvious layering.
kalitta: Fruity sweetness, citrus sweetness and sourness, tea sensation is obvious. On the palate it is full-bodied, full-bodied and balanced.
[Blue Mountain].
V60: Refreshing, dark chocolate, nutty, slightly sweet and sour fruit. Kalitta: Nutty, dark chocolate flavor, mellow taste, sweet and sour balance, comfortable.
From the flavor performance of the coffee brewed by these two filter cups, it can be seen that the conical filter cup represented by V60 is good at expressing the flavor of coffee beans, and the trapezoidal filter cup represented by Kalita is good at expressing the balance and heaviness of coffee.
The reason why these two types of bowls can produce different flavor performance lies in their structure. V60's spiral-shaped long and short alternate diversion ribs and large drain holes allow water to flow down through the diversion bone quickly after soaking the coffee grounds, and the conical structure is easier to achieve uniform water injection when injecting water. It belongs to the filter bowl that relies on water injection to flush the coffee powder to release coffee substances, except for the Kono filter cup that was born earlier, and the conical filter cup that appeared later belongs to this type. 
Although the trapezoidal filter cup also has ribs, but these vertical ribs are more used for the exhaust effect of coffee steaming, 3 small drain holes make the water speed relatively slow, although the trapezoidal structure is not conducive to uniform water injection, but the slow water speed makes the water injection do not need to be so rigorous, such a design makes the coffee most of the time in the immersion extraction, so the overall performance of the coffee will be full, balanced, mellow taste.
Returning to the question of why trapezoidal filter bowls are becoming more and more rare now, it is actually a retreat from the environment. The V60 was born around 2000 because of the need for it, when the transition from dark roast coffee to light roast coffee was the mainstream, and more attention was paid to the expression of flavor for light roast coffee. The V60 is also revered as a classic for its ability to better express the flavor of coffee (of course, marketing is also a reason). When community coffee shops with pour-over are everywhere, the ** degree of conical filter bowl increases greatly, and comparatively, other shaped filter bowls, including trapezoidal filter bowls, are also more rare. However, we can also see coffee with trapezoidal cups in some of the big chain coffee shops. This also has to say that the advantage of trapezoidal filter bowl - stability. Because of the trapezoidal filter bowl slow water drainage, soaking extraction, and reduced water injection influence, as long as the powder amount, grinding, water temperature, ratio, and extraction time are designed, the only thing the barista needs to do is to pour the water within a fixed time. As a result, these stable, easy-to-use, and easy-to-operate coffee filter cups are very popular with coffee chain brands with high mobility. - end -