It is a good question and somewhat a mystery. There are many hypothesis that can be constructed to explain this.
Toei was once good, the original Dragon Ball I considered a good animation although it is very dragged in pace, that's why they did another anime after all.
Now, however, the animation is bad, I think I don't have explain why it is bad, right? I think Shueisha is also realizing this because it is putting their new success in more responsible hands recently, the market has changed and there is no way to do long series with low budget and therefore bad animation anymore.
From 2010 to now Toei has made more Precure than anything else, this is a series for another public, much younger that are not worried that much about animation as long as it sparkles. From 2010 to now we had Toriko, Dragon Ball and World Trigger and all of them suffer of complaints about animation.
In the end, there are many things involved in the business that we don't have knowledge about, I think Toei is earning more money with what they did in the past and what they are doing now. They were lucky to be chosen, a long time ago, to make series like Dragon Ball and One Piece, that makes more money than everything else combined.