Why?
Because people abused it badly.
They'd quote a huge post just to address a small part of it. Or, worse, just to say "That's neat," or "+1" or something else completely inane.
It got really bad when people would chain quote, bringing in multiple quotes just to add a few words, something that I christened "quotarding" many years ago.
So, the quote button was done away with.
This thread discusses how to quote:
http://thefiringline.com/forums/showthread.php?t=323215