Why can modern submarines dive much deeper than WW2 era U-boats? What technology made it possible?

In this regard, there is no technological breakthrough; it all comes down to tactics and economics. Submarines were mass-produced in the hundreds during WWII, so they had to be inexpensive. Cheap meant fewer materials and a lower level of sophistication.

Today's submarines are made in small batches, one at a time, and are far more expensive and larger than traditional subs, so they get everything.

The technology of the time was already capable of producing much deeper diving subs, albeit at a cost. Stronger pressure hull=more weight=more buoyancy tanks=more airpressure to blow it all