it is actually a different domain, which is a broader classification than kingdom
anyway it is because archaea and bacteria are very different genetically
also they have some very different structures
the phospholipids in cell membrane in archaea are branched, while they are smooth in bacteria
bacteria and archea have different types of flagella
they have different ribosomes
thier cell walls are assembled differently
archaea have stuctures called hami which act like hooks
bacteria can share DNA via structures called pili
