According to Contact Music  They were exhausted.... WHAT!  

David Lee Roth posted a video online on yesterday (20May12) explaining why the rockers have pulled the plug on 30 U.S. dates . Dismissing reports he and his bandmates are not getting along, Roth says, "The band is getting along famously, better than we have in quite some time."

And he insists that the group is just exhausted and desperate for a break: "As usual, we bit off way more than we could chew when it came to scheduling. The band is winning, but our schedule has been sidelined for unnecessary roughness."

Sorry you have had a break for a couple of decades, how about sucking it up, do 30 more shows and ride off into the sunset you bunch of wimps!