score:2
I think that the answer lies in how the Church understood the nature of the Eucharist. Whereas Passover was essentially a commemoration of an event of the Exodus, the Eucharist is something entirely different. According to the account in Exodus (12:14), Passover was instituted strictly as a memorial. Christ Himself described the nature - and necessity - of the Eucharist:
John 6:53 (KJV 1900)
Then Jesus said unto them, Verily, verily, I say unto you, Except ye eat the flesh of the Son of man, and drink His blood, ye have no life in you.
I am not aware of any early Christians who maintained that the Eucharist should only be celebrated once a year in lieu of the Passover meal (though I am sure there probably were some). There are writings, however, of certain Church Fathers explaining the need for frequent (i.e. more than once per year) Communion. John Cassian (5th c.), a western Church Father with an eastern upbringing wrote:
We must not avoid communion because we deem ourselves to be sinful. On the contrary, we must approach it more often for the healing of the soul and the purification of the spirit, to show our humility and faith, by considering ourselves unworthy and in need … that we desire even more the medicine for our wounds. Otherwise it is impossible to receive communion once a year, as certain people do … considering the sanctification of heavenly Mysteries as available only to saints. It is better to think that by giving us grace, the sacrament makes us pure and holy. Such people who receive less often manifest more pride than humility … for when they receive, they think of themselves as worthy. It is much better if, in humility of heart, knowing that we are never worthy of the Holy Mysteries we would receive them every Sunday for the healing of our diseases, rather than, blinded by pride, think that after one year, receiving once or twice yearly, we become worthy of receiving them.
Conferences of John Cassian, Chapter XXI
Similarly, John Chrysostom (4th c.) commenting on those who did not receive the Eucharist at the weekly Liturgy, wrote:
Look, I entreat you: a royal table is set before you, angels minister at that table, the King Himself is there, and yet, you take no account of it. Are your garments clean? Then fall down and partake! For everyone who does not partakes of the mysteries is standing here in shameless falsity. When you behold the curtain drawn, then imagine the heavens are let down from above, and that the angels are descending! Why stay at liturgy and yet not partake of the table? I am unworthy, you say. Then you are also unworthy of that communion you also have in prayer. Come!” Holy Father
Commentary on the Epistle to the Ephesians, Homily III
These writings, although not by the Apostles themselves, might help to explain why the Eucharist was seen by the early Church as something more than an annual replacement of Passover.
Upvote:2
In fact as one comment states Jehovah's Witnesses observe the event just once a year. In the January 1 2003 issues of the Watchtower magazine on page 31 a similar question was posed by a reader. The answer provides evidence that early on the apostles did conclude that once a year was appropriate. In fact it states that only later in the second century did the custom change to more frequent celebrations.
Below is the question and answer:
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) Some feel that the word “often” here indicates that Christ’s death should be commemorated frequently, in the sense of many times. Hence, they commemorate it more often than once a year. Is that what Paul meant?
It is now almost 2,000 years since Jesus inaugurated the Memorial of his death. Therefore, celebrating the Memorial even once a year means that it has been celebrated often since 33 C.E. However, 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·laʹkis, which means “often” or “frequently.” Rather, he used the word ho·saʹkis, 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.’*
How often, then, should the Memorial of Jesus’ death be commemorated? It is appropriate to observe it just once a year. It truly is a memorial, and memorials are usually observed annually. In addition, Jesus died on the day of the Jewish Passover, which was held once a year. Appropriately, Paul referred to Jesus as “Christ our passover,” since Jesus’ sacrificial death opened the way to life for spiritual Israel, just as the first Passover sacrifice preserved alive the natural Israelites’ firstborn in Egypt and opened the way for the nation’s release from slavery. (1 Corinthians 5:7; Galatians 6:16) This connection with the annual Jewish Passover is further evidence that the Memorial of Jesus’ death should be observed just once a year.
Moreover, Paul associated Jesus’ death with another annual Jewish feast, the Day of Atonement. At Hebrews 9:25, 26, we read: “Neither is it in order that [Jesus] should offer himself often, as indeed the high priest enters into the holy place from year to year [on Atonement Day] with blood not his own. . . . But now he has manifested himself once for all time at the conclusion of the systems of things to put sin away through the sacrifice of himself.” Since Jesus’ sacrifice replaced the annual Atonement Day sacrifice, the Memorial of his death is properly observed annually. There is no Scriptural reason to observe the Memorial more frequently than that.
In harmony with this, historian John Laurence von Mosheim reports that the second-century Christians in Asia Minor were accustomed to observing the Memorial of Jesus’ death “on the fourteenth day of the first Jewish month [Nisan].” It was only in later years that it became customary in Christendom to observe it more often than once a year.
[Footnote]
Compare the account at 1 Samuel 1:3, 7. There, “as often as” (in the modern translation of the Hebrew) refers to events that happened “from year to year,” or once a year, when Elkanah and his two wives went to the tabernacle at Shiloh.