Presumably because nobody cares about the security of email addresses. The part of their system which handles credit cards hasn't been shown to be compromised, but maybe Gmail's spam filter needs to work a little harder. (I've already spent longer writing this than I spend going through my spam folder each year)
If a company's user email list is hacked, how much harder is it to attack other information? Financial information (e.g., credit cards) usually get extra security, but plenty of other information is typically stored right next to email addresses (e.g., user behavior history, IP addresses, signup dates, pricing info, password hashes, friend connections, etc etc etc).
So whenever a company says that only their user email addresses were compromised and nothing more, I'm pretty skeptical of the validity of their assertions.