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.
In the "bad old days", tables were abused to build page layouts. There were all sorts of problems with this due to weird rendering rules within table cells and the table itself.
Some people piled onto the "omg tables are so bad" fever without properly acknowledging that they are still the best way to display tabular data - their original purpose.
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.