Customer Names and Email Addresses Compromised

Epsilon, an email marketing firm for many corporations, has had it's email database hacked. Names and email addresses of customers have been leaked.

Some companies who have reportedly released notices to their customers about the e-mail breaches.
My question is, why did Epsilon put all their eggs in one basket? I have received five emails from companies that have had information breached. Here is a list of companies that have sent emails to their customers advising them of the breach:

Barclays Bank of Delaware
Brookstone
Capital One
Citi
Disney Destinations
Home Shopping Network
JPMorgan Chase
Kroger
LL Bean
Marriott Rewards
McKinsey & Company
New York & Company
Ritz-Carlton Rewards
The College Board
TiVo
US Bank
Walgreens

This isn't all of the companies, just the ones who were honest enough to advise their customers.