Because the Sabbath is Sunday, not Saturday.
The Christian can meet any time they want.....as long as they also obey God's command for the Sabbath.
But by Biblical standards that would start the evening before, so Saturday night is included as part of the Sabbath: Saturday evening until Sunday evening.
I really just don't get this, and I think a lot of people are being very snippy about the whole thing.
Uh...there's quite a few people both Protestant and Jewish who would disagree with you.