@Syanindita Short Answer: It's because it's easier to engage in something targeted to teens than adults.
Elaborated Answer: Shounen is binary stuff. You need something rather simple-ish/straight forward with tints of fanservice to catch teens/young adults (main target audience in Japan). With Seinen you don't really have that simple of a formula, mainly due to having to deal with grown ass people that won't buy into that shit and the fact that it (the genre) touches somewhat sensitive subjects that often times end up being boring (for your average teen/young adult) along with less adults reading manga than your average young adult/teen in general, hence why it isn't as popular as your #232342543 Shounen Isekai with OP MC that has a shitton of hotties drooling over his dick 24/7 type of series.