That question haunted me from day one. As JWs, we were taught to really meditate on and visualize Bible accounts, but I think that can backfire on JWs. I can't stand to visualize the flood account long, but I've done it enough to picture the horror of animals treading water until they give up.
In the past few years, I read an account of a man (I think in Florida and who I think is an attorney) who was offshore (I think several miles) in his small pleasure boat. He found out there was a raccoon that had been sleeping somewhere on the boat. He somehow (don't remember exactly) got the raccoon into the water and left him out in the ocean to die a horrible death. There was a major outrage and a huge uproar. Imagine that scenario billions of times over.
A few years ago, I found a half-grown dead squirrel in a 5-gallon bucket that was half full of water. I had left the bucket in my yard and I guess it collected rainwater. I don't know how the squirrel got in, but it couldn't get out and drowned. I was horrified, but that's nothing compared to what happened in the supposed flood.
Not only puppies and kittens. I've raised four raccoons from infancy, and all four were beloved. They were playful, intelligent, feisty, affectionate, and social. They seemed to really love life. (One was seven years old and died two weeks ago from today. We took her into a neighboring state to the only vet we could find that would treat her. We spent $760 trying to save her. It was horrible. I loved her so much and she really seemed to love me.) I can't imagine those coons as babies or adults dying a horrible death such as an animal would die in a flood. I can't imagine elephants, big cats, or anything dying like that.
I always secretly (except to my wife) hoped that Jah would resurrect those animals. I searched for clues in the Bible. I found a passage in Psalms that refers to animals' being in sheol. That gave me hope that they'd be resurrected since JWs taught that all in sheol (or hades) would be. So, I consoled myself by telling myself that maybe there was some answer that we don't know about.
Great evidence indicates that animals have been suffering for millions of years - millions of years before man appeared on earth. So, JWs can't say that animal suffering started due to man's rebellion in the Garden of Eden.
There's horrific suffering that goes on now that ruins my life thinking about it. I know I've mentioned this before on this site, but I'm going to keep mentioning it. There's a market in Asia where cats are beaten until they're staggering and then boiled alive because some people think that boiling them alive makes the meat taste better. There's a another place, also in Asia, where raccoons (or animals similar to raccoons) are beaten until they stagger and then skinned alive. I accidentally stumbled on the video and it horrified me beyond what I can describe. A raccoon, still alive, with its skin completely gone, was hung upside down by it's feet on something like a clothesline. It tilted its head up and looked at its skinless body; all it saw was muscle, etc. It was worse than anything I've ever seen in any horror movie. I could literally beat to death the person who did that.
Imagine the precious creatures in the photo you posted slowly drowning, or being skinned alive, or being boiled alive, or being burned alive. I will not be able to die in peace knowing such has gone on and still does.
Sorry for writing so much; this is a subject I think about constantly. To answer your question, I don't know why and would really like to.