If you select the bit of text that you want to reply to and then
click
'reply', Gmail will format it as a bottom-post. Try it! :)
OK, here goes. But why is not default?
The just-so story is so that one can show respect for a cultivated
audience (or at least distinguish oneself from the uneducated) by
making a little effort to bottom post, in the same way that one might
make a little effort to use standard grammar and orthography, instead
of typing whatever comes at hazard to the fingertips.
The real story is probably that when we brought the net to the world,
the world changed the culture of the net far, far, more than the net
changed the culture of the world.
cf "Pandora's Planet" (Anvil, not Cameron)
-Dave
(If I were to successfully claim "bottom posting is the sanskrit of
online discourse", would the BJP try to promote it?)