Well...actually, Western languages (I'm looking mainly at Latin-derived alphabets here, since I'm less familiar with Greek and Cyrillic scripts) exhibit a pretty wide disparity and a fair bit of change over fairly short periods, especially in terms of writing scripts. I think the greater perceived uniformity might be because of standardization that came into play with the spread of movable type. It also helps to remember that Brahmic scripts are actually abugidas, not alphabets.