For the same reason, certain short words are used a lot in headlines. For example, "rap" in place of "criticize." "GOP" (for "Grand Old Party") is a common headline abbreviation for "the Republican Party." "Dems rap GOP on taxes" = "Democrats criticize the Republican Party's position on a tax bill." The city of Boston becomes "HUB" (Oliver Wendell Holmes said that Bostonians regard Boston as "the hub of the universe" and so "hub" became a nickname for Boston): "8 Hub Collegians Safe" = Eight Boston-area college students are safe. Notice that the rule that small numbers should be spelled out is frequently broken in headlines. "Nix" means "to cancel, reject, refuse, or say 'no' to" something: "Benji's Deli & Restaurant nixes Tosa plans"--Benji's Deli and Restaurant has decided not to go ahead with plans to open a new restaurant in Wauwatosa.

To save space! In fact, it's not only articles that are dropped. Other grammatical words such as auxiliary verbs and pronouns are also omitted. When journalists write headlines they want to catch your attention and cram in as much information as possible into as few words as possible. This means keeping in all the content words and leaving out the grammatical ones.