Can anyone tell me why it died?
Brocktree ..It most likely died from stress. Stress from overcrowding..
There are too many fish in that tank ..way too many!!
They may look fine to you but they are not...a pregnant guppy need a place to go hide to have her babies ..and if she can't she will die trying to wait for a good place to go .
Please read the lists that Pat Mary gave you...there are lots of example of what can live very happily in your sized tank.
You should take some of those fish back to the store and trade them in for some that can live in your tank.
Now answer us these questions ..How often do you change water?? and how much water do you change when you do it?
their water is their toilet...
We have to change out water so that they have clean water to live in ..and not polluted water ..the stuff just doesn't go away ..even if we don't see it..
The only way to get the pollution out of the tank is to change out part of their water regularly..
One of the ways to know how much pollution is in the tank is to have the water tested.
Please read about your fish in the fish profiles part of the forum ..and please tell us about your water.