Pope Shenouda III
In making the sign of the cross, we proclaim the Lord's death for us, according to His commandment
This is the commandment of the Lord: to proclaim His death (which is for our redemption) till He comes (1Cor.11: 26). Every time we make the sign of the cross we remember His death and will remember Him till He comes.
We also remember the Lord in the Eucharist but this Sacrament is not celebrated constantly whereas we can make the sign of the cross at any time, and thus remember the Lord's death for us.