Priest and Choir

What is the reason in the TLM for the priest two read the propers separately from the Choir?

The Catholic Encyclopedia:

In other cases concelebration was in the early Middle Ages replaced by separate private celebrations. No doubt the custom of offering each Mass for a special intention helped to bring about this change.The separate celebrations then involved the building of many altars in one church and the reduction of the ritual to the simplest possible form. The deacon and subdeacon were in this case dispensed with; the celebrant took their part as well as his own. One server took the part of the choir and of all the other ministers, everything was said instead of being sung, the incense and kiss of peace were omitted. So we have the well-known rite of low Mass (missa privata). This then reacted on high Mass (missa solemnis), so that at high Mass too the celebrant himself recites everything, even though it be also sung by the deacon, subdeacon, or choir.

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