At one point people decided that tables are lame.

Iframes on the other hand posed a security issue (cross site scripting) - you have two pages living inside the same window, with possible access and trivial attack vectors like creating a page with one iframe being invisible (1x1px for instance).

But that's just an exception, frontend is really a fashion driven developement and there's often no rhyme or reason to why the mob decides to chose one way over another.