What caused unrestricted submarine warfare by the Germans?

Restricted warfare (being nice) got them sunk. Because the victim ship would radio a "May Day" or "SOS", which naturally included the victim ship's location (position). Well, this also pin-pointed the subs position! Consequently, by the time it took those people to abandon their vessel, a plane or sea vessel could or would be tracking or diving down the subs neck. Bottom line; it was dangerous for subs to be nice about it!

Un-restricted warfare was easier on the sub & easier on the crew.