Why did Germany’s allies go over to the side of the Anti-Hitler coalition precisely in 1944?

Because in 1944 the question of Germany’s defeat was removed. Everyone already understood that Germany would not be able to win, and her defeat was a matter of time. Thus, in order to hasten the defeat of the enemy, his allies came out against him. So, for example, in Italy, the British party won the elections, which immediately declared war on Germany and began to collect troops in order to take part in hostilities against its former ally.