Why were Christian festivals placed on the dates of pagan festivals?

The original Christmas was the 6th January, the Alexandrian dare of the birth of Osiris, but why move the date to Dec.25th the date of Sol Invictus , the rebirth of the sun, and of Mithras the Roman soldiers god, it is also on the winter solstice festival, Easter is a pagan festival of rebirth hence the name taken from a pagan goddess Eoestre. In fact nearly all Cristian festivals are on pagan dates.