It's true that the date for Christmas and many, if not all, of the traditions associated with Christmas were pagan. But Easter (though the name for it was adopted from a pagan goddess) is an entirely Christian holiday (even though some of the traditions are also pagan).
It's just tradition. For the first three hundred years of Christianity's existence, it was heavily persecuted, so the Christians had no way of adopting their own holidays as a unified body. When Constantine "paganized" Christianity, he was able to give them a unified body. Of course this wasn't completely a good thing.
Anyway, seventeen hundred years later, we Christians still use the holidays that Constantine helped to change over, in order to make the conversion process easier...because it's tradition, and because it helped us once to be unified. Besides, as far as Jesus' birthday, we don't even know when that was anymore. I suppose the earliest Christians must have known, but that was a long time ago. Why change the date now? It's been too long.
So that's my two cents.