The Eucharist was not generally reserved in Churches for the first 8 or 9 centuries; then it started to be reserved, but the tabernacle was not in the center of the church; that came a while later.
Liturgically, there is a conflict between the Eucharist present prior to the Consecration. Part of the action of the Mass, the Consecration, is to make the Eucharist (Christ) present; that in part is why the priest is supposed to Consecrate enough Hosts for the Mass and Communion, and not excessively more.
In many European churches, the tabernacle is not in the center of the church. Further, where there is Eucharistic Adoration, particularly 24 hour Eucharistic Adoration, it is much easier to accomplish and much more intimate to have it in a chapel; one does not have to light a large church during the night, nor heat it, and in a small chapel, one is closer to the Eucharist physically.