why don't christians incorporate prostration as a part of their act of worship?
Have you read this thread? This question has already been asked and answered multiple times. We do. We have even posted video examples.
You assert things that are not true as though they are and fail to see other things that are true as being so. As a result you have a messed up view of what Christianity is really like. But the fault isn't with Christianity, but with your inability to realize what is and isn't true.
Christian do pray by prostrating.
Christains also pray kneeling, sitting, standing, jogging, driving, parachuting, swimming, riding bicycles, and in a myriad other positions, indeed we are taught to pray without ceasing.
Jesus never taught his disciples a particular preferred posture for prayer. So, we are neither keeping nor breaking Jesus' instructions with regard how to pray when we pray.
Jesus' disciples did stand when they prayed. See Mark 11:25. Other Jews did as well. See Matthew 6:5.
You asked for a source on the prayer posture practiced by first-century Jews:
those Forms of Adoration described in the oldest portions of the Mishnah date from the pre-Christian time. About the time of Jesus there was a dispute between the Hillelites and the Shammaites concerning the proper attitude in which to recite the Shema'. The latter, in opposition to the former, who were indifferent as to posture, insisted that this prayer must be said standing in the morning; but that, in the evening, the aforementioned posture of solemn inclination was the appropriate one. This dispute lasted until nearly the end of the first Christian century (Mishnah Ber. i. 3). The chief prayer, the Eighteen Benedictions, was, however, always said standing (Mishnah Ber. v. 1; Gem. 30a). Hence the name "'Amidah" (Standing) for the Eighteen Benedictions.
Read more: http://www.jewishencyclopedia.com/view.jsp?artid=855&letter=A#ixzz1JrcysJbq
The three principal postures of the body at prayer prevalent among Jews in the time of Jesus—standing, kneeling, and prostration—were adopted by the Christians, at times to the minutest details. Among the early Christians the most customary of all the Forms of Adoration was standing, adopted from the Jewish attitude during the "Eighteen Benedictions"—the prayer of prayers. This may be seen from the numerous illustrations of that time in Aringhi's "Roma Subterranea," Rome, 1651-59. Their outspread hands and their faces turned eastward correspond exactly with the Jewish customs already mentioned, namely, with the ancient practise of turning toward Palestine, which for Jews in Europe is eastward, and with the practise prevalent in all synagogues, of placing the ark in the eastern wall. The custom of kneeling, especially in private prayer, was likewise adopted by the earliest Christians (Luke, xxii. 41; Acts, vii. 60; ix. 40; xxi. 5; Eph. iii. 14, etc.) and became general (see "Hermæ Pastor," i. 1; Clemens Romanus, i. 48; Tertullian, "Ad Scapulam," iv.; Origen, "De Oratione," xxxi.). Less prevalent in the early days of Christianity was the prostration to the ground, employed only on special occasions (Socrates, "Historia Ecclesiastica," iii. 13, 17). How completely the Church ritual of early times was dominated by the Synagogue is shown by the usage prevalent in the Christian Church, and mentioned by Tertullian ("De Corona Militis," iii.), that on Sunday, and during the whole week of Pentecost, prayer was not to be said kneeling. The synagogal custom (minhag), as old as the first Christian century, omits the prostration on all festivals and semi-festivals (B. M. 59b).
Read more: http://www.jewishencyclopedia.com/view.jsp?artid=855&letter=A#ixzz1JrdiuPdG
Jews today also pray in mulitple positions and postures, sometimes prostrated, but not always. Jews also pray both standing and seated during passover meals that I have attended.
Jesus prayed in multiple positions. It is specifically recorded that he prayed both face to the ground and also kneeling. Since Jesus disciples and other Jews are reported to have prayed standing, it is likely that Jesus did as well and it is not specifically reported because this was the most common way to pray (see above material from the Jewish Encyclopedia).
HOW MANY DIFFERENT POSTURES IN PRAYER ARE MENTIONED IN THE BIBLE?
1. Sitting (Acts 2:1-4 [Pentecost], Elijah under a broom tree - 1 Kings 19:4)
2. Dancing (2 Samuel 6:14)
3. Head bowed (Luke 18:13, Gen 24:26, Acts 21:5)
4. Publican smoke his breast (Luke 18:13)
5. Standing (Mark 11:25)
6. Prostrate on the ground (Joshua 5:14; Matthew 26:39 [Christ])
7. While lying sick (Psalm 4:4, 63:6; 2 Kings 20:1-3)
8. Kneeling (Daniel 6:10)
9. With hands raised (1 Timothy 2:8)
10. Elijah stretched his body over a widow's dead son
11. Stephen prayed while looking into heaven (Acts 7:54-60)
12. Peter prayed while walking on the water (Matthew 14:30)
13. While hanging on a cross (Luke 23:42)
14. Jonah prayed in the belly of a fish (Jonah 2:1). We can only imagine what bodily posture that would have required.
15. With eyes open (Christ - John 17:1, 11:41, Matthew 14:19)
16. Praying with their mouth moving (Hannah - 1 Samuel 1:12-13)
17. In stocks while in prison (Paul & Silas - Acts 16:24-25) 18. Hand laid on other people (First 7 deacons - Acts 6:5-6; Christ laying hands of little children - Matthew 19:13-15) 19. Clapping hands (Psalm 47:1)