Why was Saint Christopher desainted?

He wasn't. If you get a book of Catholic Saints, you will see that there are dozens of saints for every day of the year. The official church calendar or Ordo is what is used to determine which Office and Mass are said on any particular day of the year. During the reforms of the calendar following Vatican Council II, one of the things they did was to try to "prune" the calendar. St. Christopher lost his day in the Ordo for several reasons, but he certainly didn't lose his sainthood!