Hello, Welcome to the forums .
Very sorry for your loss.
From what I know there are probably one of two reasons the guppies got sick and the zebra danios did not.
#1 your water may be too soft
Guppies need hard water to do well, often the most symptom of incorrect water chemistry in hard or soft water fish is opportunistic infections brought on by the resulting stress.
Zebra danios, on the other hand , are happiest in soft water, but are relatively tough in that respect and do well in a wide range of parameters.
This toughness brings me to
possibly #2 Incomplete cycle.
NOTE: If you have already Cycled using instructions similar to these
https://www.fishforums.net/threads/cycling-your-new-fresh-water-tank-read-this-first.421488/
, then feel free to disregard the rest of the post.
Zebra Danios are in general more resistant to the ammonia and nitrite spikes that often come with a new aquarium.
It can take 30-40 days for a biological filter to fully establish in an aquarium, and despite what some websites and even pet stores claim, the process does not start without an ammonia source.
Also, bacteria in a bottle products still tend to have a delay, usually only halving cycling time at best.