Here's why:

* Abundance: Bacteria are found in virtually every environment on Earth, from the deepest oceans to the highest mountains, and even inside of us!

* Diversity: There are an estimated 5 nonillion (5 x 10^30) bacteria on Earth, and they exhibit incredible diversity in their metabolism, shapes, and functions.

* Size: While some bacteria are smaller than viruses, the vast majority of bacteria are larger than other types of microorganisms like viruses and fungi.

While other groups like viruses, archaea, and fungi are also incredibly important and diverse, bacteria dominate in terms of sheer numbers and global impact.