Good question OP. I'm sure it has been answered well.
The reason why is because Christianity, as it is presented, is a hybrid-Roman religion. The religion that the Old and New Testaments profess is not Christianity. In fact there was no single name for it during the Messianic period and afterwards. See Acts 24 for reference. After the Hebrews were widely persecuted in Rome and surrounding areas, the popular 'new' Messianic belief in the Jewish populations (original Jewish -black, who are modern day African Americans) began to catch hold in European circles. It is here when the religion began to change.
To suit the needs of pagan Europeans and as actual Hebrew populations dwindled, Euro styled 'churches' that were completely controlled by non-Hebrews started to reinterpret new testament texts. Stuff they didn't like they left out. See Justin Martyr and Marcion of Sinope.
They added and left out, eventually adding Christmas, Easter, White Jesus, Sunday (for the sun god) and removing the biblical holidays, the Sababth, the dietary laws and eventually the actual Hebrews themselves.
What we are left with is called Christianity and resembles nothing of its previous and meant worship style, by design. This is seen because today we see this happening in microcosm with many things, blues, jazz, rock, hip-hop, and modern Hebrew (again African American) culture.