Answer:
Because they do not live in the extreme environments.
Step-by-step explanation:
Cell membrane has a vital role in separating the external environment of cell from its external environment. Just like eukaryotic cell, cell membrane of bacteria is also composed of proteins and lipids (phospholipids).
If we compare the structure of ancient bacteria's and modern bacteria's cell membrane then we notice that in archaebacteria or ancient bacteria the phospholipids of cell membrane are linked with ether (means they have glycerol backbones). whereas in modern bacteria they are linked through ester linkages.
So, the stronger glycerol backbones give the archaebacteria more stable cell membranes through which they are able to resist harsh environmental stresses like salt lakes, volcanic stress, boiling mud and hot springs. As modern bacteria do not always have to live under such stresses thats why they donot have ether-linked cell membrane.
Hope it helps!