Because those six panels are usually one page in a manga book. 5-7 chapters is the norm because of that.
probably because it's around 6 chapters/episodes that can be published in a regular volume, so if the manga will not have enough chapters for 2 volumes, they try to close it 1 one volumes. now for the reason to end it so abruptly there could be so many reasons like the manga didn't get good reception or the mangaka has other works at the same time, it could be anything really. (●￣(ｴ)￣●)
editors also contribute to shorter stories.. because they have a rough outline of how many pages and chapters and a lot of times aren't working to go over those estimates (depending on the publication format). it's frustrating, and I totally agree, the ending was rushed and we really didn't get too enjoy the two main characters being together.
wtf? so much build up for so little pay off.. can someone explain to me why lots of manga seems to finish at around 5-7 chapters? this story would’ve done well with a LOT more fleshing out. It was so unsatisfying to read that i feel like there’s something stuck in my throat.