Why Is Memorial of Christ Observe Annually?

Jehovah's Witnesses observe the Memorial of Christ’s death (also called “the Lord’s Evening Meal” - 1 Corinthians 11:20) because Christians are commanded to. Jesus said to "Keep doing this in remembrance of me.” (1 Cor. 11:24)
But how often? The Memorial of Jesus’ death is truly a memorial, and memorials are usually observed annually.
Additionally, when Jesus was on Earth, Jesus celebrated the Passover with his apostles, and then instituted the Lord’s Evening Meal. Because this meal replaced the Jewish Passover it is appropriate to observe it just once a year.
Does Paul's use of the word "often" mean that the Lord’s Evening Meal should be celebrated more often than once a year? Referring to the institution of the Memorial of Jesus’ death, Paul wrote: “As often as you eat this loaf and drink this cup, you keep proclaiming the death of the Lord, until he arrives.” (1 Corinthians 11:25, 26) So what did Paul mean here? Did he really mean for Christ's death to be commemorated frequently - more often than once a year? Notice this excerpt from the w03 1/1 p. 31; Questions From Readers; par. 2: "In the context of 1 Corinthians 11:25, 26, Paul was discussing, not how often, but how the Memorial should be observed. In the original Greek, he did not use the word pol·la′kis, which means “often” or “frequently.” Rather, he used the word ho·sa′kis, which means “as often as,” an idiom meaning “whenever,” “every time that.” Paul was saying: ‘Every time that you do this, you keep proclaiming the death of the Lord.’" 
See also Memorial Celebration