The only reason that kindergarten isn't considered preschool is that since it is public education and thus free. With it being free, the vast majority of the parents decided to use it as free daycare. With the vast majority of kids being housed in a public schools prior to 1st grade, it started to be considered the 1st year of school.
Well, yes... except for the fact that there is actual instruction in kindergarten, and the kids actually learn stuff, unlike any daycare I've ever seen. Oh, and the fact that every study I've ever seen that has looked into the issue has found a significant correlation between kindergarten attendance and long-term academic success; again, unlike daycare.
So, really, once you look at the actual facts, it seems more appropriate to consider kindergarten to be school rather than daycare. Because it is.