The easy answer is money.
The more complicated answer is partly because of how comic's as an entertainment media works and partly of the theme and/or story of that said comic. In this case, if the story is about two character love each other and when they finally have sex is the end goal, then...
Isn't the manga got the axe before? Maybe ch36 is just the rushed ending and after it got continoued again, the author just retcon the first part and turned into an ntr.
The most funny and interesthing continuation of the story would been if Chisaki would doing something similiar to Ueno as what Tsubami done to him while she is under the bad witnessing it.
It's not just the manga, the twitter webcomic also not featured Ai-cha and Onii-san for god knows how long. Previously there were lot of pages were tons of progress happened between them and tons of suggestive thing either. This serialization killed the tawawa I liked.
Eh, they done it at the end, but it was such a let down and rushed that it really dissapointing. I feel like this manga lost it's plot after the author just started introducing new girls to accidetal blowjobs for Nezu, until that point it had a nice build up for Suzu-nee teasing Nezu while...
This manga is just essentially a mindless filler with low-tier erotica.
It starts out as a funny student-teacher romance story, but then it fades away.
In the official manga they barely had any progression, we never saw any of the happenings between them which was in the twitter pages, the small set up Himura made in terms of their relationship never got anywhere and basically got written out.(the part where Volleyball-chan saw AI-chan sending...