Generally speaking, in brief, Democracy. Technology. Changes to how warfare was conducted. Fiat money. Turning away from God.
Under monarchies governments couldn't leverage the kind of manpower possible under democratic regimes. And those ordering the wars often participated in and even led them personally, unlike modern democratic regimes. They had more skin in the game.
Industrialization and mechanization, of course, led to bigger, more lethal, weapons capable of greater destruction.
It also became acceptable to target civilians and non-combatants, unlike how warfare was conducted by the West in prior times. Witness the U.S. War Between the States (a.k.a. the "Civil War"), and World War I, culminating in the nuclear bombing of whole cities, Hiroshima and Nagasaki, during World War II.
Governments no longer being bound to spending REAL money (i.e., the gold standard), through the creation of fiat currency and the use of income taxes, also enabled governments to prosecute wars like never before.
One could add the so-called "Enlightenment" and secularization as deeper causes...