Because Azerbaijan’s terms for ceasefire since the beginning was that Armenia would leave the surrounding districts immediately. Azerbaijan had already regained Cabrayil, Fuzuli, Zengilan, Qubadli and parts of Lachin. When Azerbaijan liberated Shusha, Armenia agreed to meet Azerbaijan’s demands and immediately return Agdam, Kalbajar and Lachin districts as otherwise Khankendi («Stepanakert») would have been captured by Azerbaijan in following days.

Armenian side said as much. If you are Armenian then you are already familiar with the statements of Pashinyan and others following the ceasefire.

As for the remainder parts of former NKAO under Rus Pk control (2/3 of NKAO), it will re-join Azerbaijan eventually one way or another. For example cultural autonomy might be an option.