The reason why long pepper is not as popular now is due to a rise of New World chili peppers as well as logistics of early trade.
After cheaper chilis provided similar heat and black pepper also had became widely available through sea routes, the more expensive as well as difficult to farm long pepper had faded away from Western Kitchens.
When explorers had brought chili peppers from the Americas to Europe in the 1500s, it had changed the spice market entirely.
And so chilis were also easier to grow in diverse climates, which made them drastically cheaper.
And they offered a very comparable pungent kick, which essentially replaced long pepper as a heat source.
Black pepper also known as Piper nigrum grows on sturdy vines and had become much easier to transport in bulk quantities, through maritime trade routes.
This is what made black pepper much more affordable and much more accessible, which pushed long pepper out of the mass market.
Long pepper is also much harder to grow outside of it's native, specific tropical regions, mainly in Indonesia and India.
And unlike the standard peppercorns or chilis, it has also never been massed farmed successfully across the globe.
Long pepper is also composed of tiny fruits that are fused together into a small "catkin".
And long pepper contains more moisture than round peppercorns, which makes it harder to store and it can also be physically tricky and difficult to grind in a standard pepper mill, which makes long pepper difficult to process.
Although long pepper is still used heavily in traditional Indian medicine as well as in South East Asian cuisines and you can often find long pepper in specialty spice shops or even sometimes on Amazon in Spices or at local Indian grocery stores.