And then then content became frozen. There is a whole list of 20th Century topics that have been by and large rejected in today's mathematics classroom. It's time to get rid of it and open the door to some fresh, important material. They just go on tinkering with what was done the year before. It seems the central committees do not trust their own the initiatives of individual faculty members, so they shackle them with obsolete material.
Publishers respond in kind.
And the publishers stay away from texts for modern courses because new, modern, original texts are unlikely to sell well. This is the reason that most well-selling traditional calculus texts are clones of George Thomas's calculus course of the 1950's. The trend is for engineering, biology and science departments to begin teaching the mathematics their students need. Mechanical engineering departments are teaching lots of advanced calculus and differential equations. Electrical engineering departments are teaching lots of probability and complex variables.