Survey Says: Email Customer Gets Worse

It costs so much money for companies to receive and place customer support telephone calls, why aren’t more using email customer support, more effectively?
The Hornstein Associates’ annual e-mail responsiveness survey shows a decline in customer service response rates for the fourth year in a row.
The annual survey is conducted through marketing expert Scott Hornstein’s consultancy, Hornstein Associates. 38 respected companies, including such notables as IBM, Dell, Apple,, Barnes&, American Airlines, L.L. Bean and the Direct Marketing Association, have been sent the following one sentence email for the past four years:
What is your corporate policy regarding the turnaround time for e-mails addressed to customer service?
Hornstein’s survey found that in 2005 only 42% of companies responded within 24 hours, down a full third from a high of 63% in 2002.

I’ve sent email to Palm One on several occasions and their email response has been VERY good – often the same day.
Why don’t other companies realize that customer support via email is low cost and can be a competitive differentiator in their businesses?