Quite simply, fame. Katanas are much more famous than jian. The reason they are more famous is because of the association of Katanas to the heavily romanticized Samurais. Whereas the Chinese jian and its wielders do not have much modern represantation, espicially overseas.

Also, reddit is primarily an English website, frequented mainly by Americans, Europeans, Australians, Indians, etc. If Reddit was more famous amongst Chinese internet users, and had a thriving native Chinese community, there would be much more discussion about them.

Let's come at this from a Western cultural background, as we're discussing the fame of various swords in context of an Anglophone message board.

On one hand, we have the Chinese jian, imperial extraordinaire and comparable in many significant ways to European cruciform swords. On the other, we have the Japanese katana, which is both somewhat "alien" to the Western perception of swords (which are overwhelmingly cruciform or otherwise straight in pop culture) and highly praised by Eastern martial culture. You can see how the relatively "domestic" jian might be less exciting to the Western eye and consciousness than the "exotic" katana.

On top of that, however, mid-late 20th century relations with each country have been substantially different in the West. Between the true failures of communist-monikered states, outright Anglophone propaganda, China's comparatively authoritarian position and Mao's brutal regime, China alienated itself from the West. That alienation is only really beginning to end, with China and the West at something of an economic understanding while remaining ideologically opposed. Recently, a handful of Australian politicians (whose names escape me currently) advocated doubling the Australian military budget. Their reasoning? The threat of Communist China. Whether valid or invalid, that prominent political actors are still willing to bare their teeth against the now aged and feeble idea of communist assault on Western representative democracy and the capitalist system says a lot about the political and ideological gulfs that still exist. On the other hand, China has media export to the West primarily through its martial arts cinema, reaching the apex of its fame during the 80s -- roughly the same time as Japanese media export reached the height of its popularity among mainstream Western audiences.

Japan, however, has had much closer relationships with Western powers after World War II. There are many reasons for this, but from the perspective of popular media, Japan established itself as a relevant player in pop culture consciousness as early as the 50s. Films such as Seven Samurai and Gojira found relatively quick Western releases, and were praised worldwide for their sense of austere compassion; these were films that approached gross injustices with minimal restraint or fear, earning the respect of both critics and mainstream cinema audiences. Japanese cultural export would become so relevant, in fact, that Star Wars was significantly based upon Japan's "samurai" genre. Star Wars is easily one of the most influential films in Western history, informed just as much by the ideologies of foreign cultures as the pulp cowboy fantasy.

Japanese cultural influence on Western pop-culture reaches its relative apex in the likes of Seven Samurai directly and in Star Wars by a degree of separation. To this day, however, Japanese media exports have continued to be moderately relevant, moderately popular consumer goods in Western countries. Between the nerd culture mainstays of anime and video games (and keeping in mind that post-70s video games almost completely owe their existence and shape to Japanese games), Japan has continued to remain relevant to at least a segment of modern culture -- people roughly yet to come to middle age, with "nerdy" interests. However, as modern nerd culture expands (as evidenced by the overwhelming popularity of recent Marvel blockbusters, for instance), the influence of Japanese media culture and (to a more moderate degree) martial culture does the same.

In essence, what all this means is that Western people are more aware of some elements of Japanese culture when compared to Chinese culture, especially when it comes to popular media, niche media, and martial matters. Particularly throughout the 80s, 90s, and some of the post-millennial decade, there was a pop-cultural belief in the complete superiority of the katana as a sword. Some of that belief lingers today in less informed but martially invested social circles, but it's an otherwise discredited idea; the katana is factually considered to be sword like any other in overall quality, even if it does have some distinct, unique, and even remarkably clever design elements.

What does this all boil down to? Japan's closer, more amiable relationship to Western powers assisted in establishing cultural export, which has over time expanded, contracted, and altered its demographics. This means that Japan has had some degree of cultural influence over the Western arts since the 1950s, if not sooner, whereas China remained more culturally estranged from the West for a longer period of time. People are probably more interested in the katana because of its pushed reputation by way of Japanese enthusiasm and the Western disciples of that. Had China and Japan been in reversed positions artistically and politically at the end of World War II, it might well be that Western people with an interest in exotic weapons would more often discuss the jian, dao, or variants thereof than the katana.