Why are scientists who are searching for life on other planets studying Archaebacteria

On a very simple level, the answer should be that archaebacteria developped in climates, atmospherical conditions, temperatures, physical and chemical environments that were in some ways different from those that characterize Earth today. So, it is reasonable that planets with conditions like those of Earth when archaebacteria developped might bear some form of life similar to archaebacteria, or evolved from those organisms.