I think the band's core audience, when it comes to new music, isn't a singles only -type of an audience. I guess younger people get hyped about singles and that model works well for new artists that have a fanbase of young people. But Guns is an old band and their main audience are still into albums. Releasing only singles is a wrong MO for Guns, in my opinion.
An album would be an event for the fans, the singles are a bit like "yeah, cool, whatever".
But we all know who is the hold up It ain't Slash, who just released an album. It ain't Duff, who released one last fall.