i) Why is the admission of divorced and remarried Catholics to communion such a big deal, anyway? That's a bit complicated to answer.
From an outsider perspective, most Protestants don't think marriage is indissoluble. There is clearly one, and arguably two grounds for divorce in the NT. And some Protestants think there are probably more.
In addition, even if the second marriage was sinful, Protestants generally think that effectively dissolved the first marriage. So it becomes a question of contrition.
ii) On paper, there are huge differences between Catholicism and classical Protestantism. Just consider Trent or the Marian dogmas. Yet modernism has eroded the foundation for traditional Catholic dogmas. Like a broken water main that washes out the foundation of a road, the road may still look solid on top, but that's deceptive.
Why would bishops still believe in the Assumption of Mary if they doubt the historicity of Christ's Ascension? Why would they still believe in the Immaculate Conception or perpetual virginity of Mary if they doubt the historicity of the Virgin Birth? Why would they still believe in Marian apparitions if they doubt the historicity of the post-Resurrection appearances? And so on and so forth. They can't erect a wall between their skepticism regarding the Bible and traditional Catholic dogmas.
iii) So that throws emphasis on externals that demarcate Catholicism from its rivals. Unless there's something distinctive about a denomination, what's the justification for its separate existence? Why not consolidate or go out of business?
iv) Apropos (iii), the Mass is the central sacrament of Roman Catholicism. Although Rome has seven sacraments, the Mass is the most corporate of her sacraments,
[by which communicants] participate with the whole community (1322).
1324 The Eucharist is "the source and summit of the Christian life."136 "The other sacraments, and indeed all ecclesiastical ministries and works of the apostolate, are bound up with the Eucharist and are oriented toward it. For in the blessed Eucharist is contained the whole spiritual good of the Church, namely Christ himself, our Pasch."
1327 In brief, the Eucharist is the sum and summary of our faith.
As the showcase of Catholicism, who's permitted to participate becomes all-important. That's a defining sign of Catholic identity, in contrast to Protestant bodies.
Of course, enforcement has been slack for decades. So the real question is whether to make the practical status quo official.