The tabernacle is placed in a side chapel in some European cathedrals because they have many tourists who might fail to properly reverence Christ in the Eucharist if the tabernacle were on the main altar, because tourist traffic could disturb the prayer of those adoring Christ in the Eucharist, and to allow priests easier access to the tabernacle. The situation of historic, European cathedral churches does not reflect the preferred arrangement for ordinary parish churches.
History’s preferred arrangement, and the cathedral exception, is summed up in canon 1268 of the 1917 Code of Canon Law, which states:
§1. The most holy Eucharist cannot be kept continually, that is, habitually, except on only one altar of the church.
§2. It shall be kept in the most excellent and the most noble place of the church and therefore regularly on the major altar unless it seems that the veneration and cult of such a sacrament is more convenient and decent elsewhere, observing the prescriptions of liturgical law, which pertain to the final days of the great week.
§3. But in cathedral churches or in collegial or conventual ones in which choral functions are conducted at the main altar, lest ecclesiastical officials be impeded, it is opportune that the most holy Eucharist not regularly be kept at the major altar but in another chapel or altar.
§4. Let rectors of churches take care that the altar in which the most holy Sacrament is reserved be decorated above all the others so that by this appearance the faithful be moved to greater piety and devotion.
As §§ 2–3 make clear, the considerations that lead to a side chapel placement of the tabernacle are the “more convenient and decent” veneration of the sacrament and the impeding of ecclesiastical officials needing to access the tabernacle. These are not presumed for an ordinary parish and, more importantly, a side chapel is not the preferred placement under this regulation. The main altar is recommended in §2.
It is a historically flawed argument to claim that the fact the arrangement of some European cathedrals reflects the preferred placement of the tabernacle. As the former Code of Canon Law illustrates, the placement of the tabernacle in these cathedrals was the exception, not the rule, and not the preference.