In 2008, the app store was launching, and physical software was still sold at Targets, Walmarts and other large retailers. A 30% margin was roughly what retailers would make off of physical software sales. By setting the App Store to be the same, Apple was signaling to retailers that they were not trying to undercut their margin, and keep a healthy relationship with them.

It was 2008; "big box" software was largely seen as obsolete to the vast majority of developers. Marketing was done online, and the benefit of investing in retail had stopped outweighing the consequences. Online updates quickly became the norm, and service features supplanted point-of-sale business model (much like Apple's double-dip into microtransaction profits).

Apple chose 30% because they knew they weren't a retailer. You can hunt for a cheaper Diablo II copy online or at Wal-Mart, but not on iPhone.