They could but after all, the creators alone will decide on their pace of work, rushing them is never good.
Honestly, working on a one-by-one basis won't make getting those new content patches out any faster than they would this way in my humble opinion.
Better them work this way than not working at all.
And people who like (insert character here) wouldn't get their fav character's update if we focus on one route. And they are related to each other, too.