This tiny fern has the largest genome of any organism on Earth

Very good question!

Plants have more complex genomes because they are stuck where they're at and their cells have to know how to deal with every possible kind of stress they will likely ever experience. A plant has to know how to protect itself from drought, excess heat, excess cold, bugs eating it, viruses, etc. Everything its cells need to know to ensure its survival must be programmed in its DNA. That said, you can't judge plants to be superior simply because they have more complex DNA.

There is a certain elegance to simplicity and animal DNA is simpler because animals have the ability to move, and they can learn and adapt themselves to their environment. Animals aren't generally immobile. If they're hot, they can simply get up and move to find shade. If they are lacking nutrients, they can move to find food and water. If they're cold, they can move into the sun, or seek shelter. If they're hot, they can find shade, pant or sweat to regulate their internal temperature. Some animals also have the ability to migrate, while others can slow their metabolism to endure harsh winters, etc. And the more complex the animal is, the more its behavior is learned as opposed to being pre-programmed in dealing with these things as well as threats from predators, parasites, etc. ensuring its survival.

Capt Jeff S. - Loved that summation Captain. I took an evolutionary Biology course - but it was so long ago Families and Genus were still determined by looking and not genetics! My tree of life was warped.

Would that be the huge fungus living under ten countries?
Or is it the giant connected forest in our NothWest?
Or is simply you showing off your spelling skills?