Why did the meringue deflate and go soft

Meringue powder is a pasteurized egg product - "dried egg whites with stabilizers and anti-clumping agents such as cornstarch".

It is used to stiffen recipes and I think there are two main reasons for using it (as opposed to egg whites):

1) Reduced risk of salmonella

2) Less time/labour/effort used - you don't have to beat the egg whites for a long time to stiffen them

I would guess as well that it would have more consistent results. When making meringues with egg whites you have to be careful not to get any yolk or other fat into the mixture or it won't stiffen properly. There is no real risk of this with meringue powder.

Acids allow more air to be beaten into a meringue. In order to make meringue, the proteins in egg white must be denatured. In their natural state, the proteins are curled up into tightly packed balls. When the egg is beaten, they uncoil into long strands.

These strands then begin to coagulate, or join together, with the help of the sugar you add. The air you whisk in gets trapped between these joining strands, giving the meringue its characteristic light texture.

Acid delays coagulation, which means that there is more time for air to get trapped in amongst the proteins, resulting in a lighter meringue.

The acids usually added to meringue are white wine vinegar, lemon juice, or cream of tartar. Fresh eggs are more acidic than old ones, so these help too.

Some cooks use copper bowls to make meringue, because copper ions from the bowl bind to a particular protein (conalbumin) and strengthen it.