I'm pretty sure the author changed the tone because the obvious slapstick in the series had been done to death. Instead of recycling the same old jokes over and over, the tone was changed so that the fans would stay entertained from a different aspect.
Also, the host magazine may have given him the heave ho, and he only had 15 or so chapters left in his tenure, so he decided to close out the plotline about Tanaka's past. Getting magazine slots for a manga can be quite brutal in Japan, and mags have got to make tough choices sometimes.
The manga did well as a webtoon, so if the author comes up with a strong plotline he wants to write, we may see the series come back in that manner... There is a lot to close out, by the way.
What happens to Nanami and her gang; who is Mayami in love with; who is Kiribachi in love with; what is the future for Shiratori and Hijiri; How are A-ko and B-ko going to handle what happens to Reina; and the raws don't really indicate what will happen to Manabu and Reina, although this is conjecture, since I don't read Japanese.