Direct Answer
The practice assumes communion is holy and spiritually dangerous to take lightly, so the church must guard people from partaking “unworthily” and profaning “the sacred body of the Lord.” This collides with modern habits that treat communion as casual and open to anyone without testing repentance, baptism, or obedience.
Support from Transcript:
What does it say about our view of holiness if we fear excluding someone more than we fear profaning the table? How might repentance and restoration look if communion is treated as sacred, not routine?