Drinks were served on flights until September 11th, 2001, when extra security became necessary to ensure passenger safety during air travel. With the new regulations in place for air travel, passengers cannot claim their carry-on bags until after passing through security checkpoints. This may be inconvenient for some passengers who want to drink on the flight if they forget to bring an unopened beverage with them for consumption on board.
This is not just a U.S., issue however; international flights also impose these new regulations starting in 2012 (particularly of concern since alcohol was still being served for many years before this change). Airline staff members will not be allowed to serve any alcoholic beverages either during pre-flight or post.