Politico:

“Disclosure was very important to us, which is why the 527 was created,” Forti said. “But some donors didn’t want to be disclosed and, therefore, a (c)4 was created,” Forti explained, referring to Crossroads GPS.
Forti’s frank explanation differs from that previously offered by the Crossroads team, which had asserted that they always intended to create a 501(c)4 because it was better suited to facilitate issue-based advocacy.