HeroAca's first chapter split was a conscious decison by the director Nagasaki Kenji.
http://www.excite.co.jp/News/reviewmov/20160623/E1466636924330.html?_p=3
In this interview, he says that he purposefully made it into two episodes in order to deliver the full impact of the Izuku running in to save Bakugou.
In the manga, everything works because of the medium which is inherently different but for anime, the buildup has to be created. Nagasaki says that stuffing everything into one episode would have cut away from really depicting the situation that Izuku jumped into. The other heroes did not move in because they had to create a situation that no one else would get involved in the fray and they themselves were unsuitable to deal properly with the Sludge villain. All Might also was at his daily limit.
All this can be shown pretty easily in a few pages in the manga but it need to have the impact in the anime. Nagasaki says that by splitting it, he feels that this pinch situation was conveyed better. Izuku jumping in and moving All Might to go past his limit and put in one more punch had more feeling and the final scenes also had more emotional impact.
I'm pretty sure that the limit of 13 episodes was also there but the emotional impact took preference over putting everything into one 25 min episode. Its worth noting that in the re-screening events, they always screen both of these episodes as a set. ALso, Nagasaki got the offer to direct one year before airing but at that point, he didn't really know it was Heroaca. He then went on to read all the volumes and then began planning out how to adapt it. This is pretty much standard in the industry.
Also, Shueisha isn't the only one who plans how these things go about. If Soma gets a 3rd season, Shueisha will probably be completely on board as it still sell very well. People have this feeling that 500k+ is the marker of a good selling series. Series that sell around 200-300k are just as good comparing how many titles come out every year. Soma's other merchandise, figures, etc. also did pretty well. Its still is in the top 30 franchises in 2016.