The proper sanction for heresy is major excommunication; Pius XII knew that, even if his successors often seem not to. If Fr. Schillebeeckx was a heretic, why was he not excommunicated?
Why were progressivists like Schillebeeckx not excommunicated for heresy by Pope Pius XII? While they harbored heresy in their hearts, they cleverly disguised it so as not to be crushed. They believed their crusade of auto-demolition was a noble cause. They were not stupid about it, either. They meticulously wormed their way into positions of power so that when the time was right, they could all strike together.
You might as well ask why Pope Pius XII brought in someone as destructive to the traditional Church as Annibale Bugnini, the "chief architect of the liturgical reform." Why would he place someone with such an agenda to uproot tradition, into a place where he could do just that? If it wasn't heresy, it was as close as he could dare get and not get excommunicated.
Unless you are aware of how consequential the errors of theologians and teachers of religion can be, it is useless to discuss this issue. If, for example, you do not think that the destruction and confusion that followed the Second Vatican Council is something we ought to find appalling, there is not much to say. Perhaps you think there has been progress. Within varying degrees among them, progressivists recognize destruction as good and necessary so that their new religion may grow and conquer. Do you agree with them?
Available quotations from avant-garde “luminaries” such as Fr. Schillebeeckx and his comrades make it clear that these men believed that the “old religion” (or at least parts of it) had to be wiped out in order for the “new springtime” of Vatican II to succeed. Is that what you think, too?
Pope Pius XII did a lot of good, but he also, unfortunately, allowed Modernism to continue to grow. He permitted and approved the first changes to the Mass, the reactions to which the revolutionaries observed, and since Catholics the world over did not cry out in opposition to those first changes, the revolutionaries rightly concluded that more drastic changes would not be resisted, either. They were mostly correct. That doesn't mean they were morally right in making more changes.