because detente ended..
Reagan came into power in the US, hyped up the anti commie rhetoric, upped the ante in propaganda and the arms race
the hawkish elements in the Kremlin saw the US as weakening in their stance on containment, as the US supported Shah in Iran was outed by teh fundamentalist Ayatollah, which also lead to fears of the same thing happening in Afghanistan, so they abandoned economic reforms to fight war against the MUjahideen fighters in Afghanistan, which is all part of the US sphere of interest, outlined in both the Eisenhower and Carter Doctrines.
Carter pissed the USSR off by lecturing them on human rights, also appeared a soft target, bit naive, especially after US loss in Vietnam.
The US didn't ratify SALT 2, relations were worsened and the arms race started up again..
i dunno exactly how to answer that question, apart from just explaning why detente ended..