I think those (like brokers) who retain stories longer than a news source's regular time limit pay a higher fee. How much disk storage one wants to dedicate towards archives also plays a part in the decision making.
This is from Yahoo:
How Long We Archive News
News stories are archived for a limited period of time, as required by the various news sources:
* Reuters - 30 days * Standard & Poor's - 90 days * Associated Press - 14 days * PR Newswire - 90 days * Business Wire - 90 days * Canadian Corporate News - 90 days * The Motley Fool - 90 days * TheStreet.com - 90 days * Internet Stock Report - 7 days * The Online Investor - 90 days * Individual Investor - 90 days