Why, we may ask five decades later, were they reduced to spectators at the act of baroque theater that had become of Mass—the "opera of the poor," as Voltaire put it? Yes, medieval English, French, and Italian were closer to Latin, but not so much closer that the faithful were so drastically less capable of comprehending the liturgy just a few centuries later. Consider what the faithful did during Mass in the first millennium: on great feasts there would be a night-long vigil, beginning outside the church. The pope or local ordinary would arrive for the Office and a procession would follow into the church proper. Then Mass would be celebrated. In the middle ages, there would be prayers and hymns around the church to the local saints, a procession would follow around the church and arrive at the rood screen, where prayers for the church were made in the vernacular, and then Mass would follow. In both cases the faithful stood unimpeded and were free to move about as the Spirit or their bodies compelled them. Those tired could take a break. Those wishing to pray quietly could disappear to a side chapel. Those especially moved could stand closer to the front. The call to Communion would been the movement of a crowd towards Christ, not unlike those who sought to hear Him preach on the coast on atop the Mount. Communion was not a linear parade.
How different was Mass with pews? First, the ritual itself was highly reduced: no processions, no night watches, no rites for the local saints. Just one or three clerics performing the Mass itself in a simple fashion. Then there was the pew. The faithful could not move, could not process, could not go somewhere to pray, could not do anything but sit still and watch for an hour. The only "break" in action was the possibility of a sermon. Gone were the rood screens and the air of mystery. Instead, an elevated altar behind a rail was viewed by a layman sitting on a bench. There was the mystery, to be watched by a remover spectator in plain sight.
As low Mass replaced high Mass as the norm, people were naturally further disengaged. Even in languages closer to Latin than our English—French, Italian, Spanish, Portuguese—the faithful could not really do anything other than find means of diversion, means of entertainment.
If anyone doubts this thesis, and you are open to doing so, consider the 1955 Holy Week. Eight years before Sacrosanctum Concilium put "active participation" into our liturgical vocabulary and fourteen years before the new Mass ritual, Pius XII introduced a series of rites which have, as one of their stated goals the "living participation of he sacred ceremonies." Churchmen in 1955 understood "living participation" differently than churchmen in 1455 did. They wanted the priest to talk to the congregants and for the congregants to talk back to them, and what better way of doing that than facing them? Pius's Holy Week includes Palm Sunday (now "Second Sunday in Passiontide") and Holy Saturday (now "Easter Vigil") rites conducted at length atop a table versus populum in front of the actual altar. Ritually, it was likely an experiment to test how the faithful would receive it and to tweak the ceremonies for when the complete reform was ready for release.
Mass facing the people, what Geoffrey Hull called the "great narcissism," became a matter of fact in almost every parish by 1965.
Pews were not the immediate cause of the new liturgy, but they were a necessary cause.