[mywittyname]

As it was explained to me by my musician aunt (and I've heard it repeated in other places): because Music Execs. Lots of successful pop music is uses the 1-4-5-(6) chord progression, thus, when executives are picking hit songs, they go with what they know will work.

It's like bringing brownies to a potluck. It won't blow anyone's mind, but everyone will happily eat them.

[fuckf4ce]

That still doesn't explain why those chord progressions became popular, and they far predate the late 20th century record industry, so it really isn't a very satisfying or informative explanation, even though it's not outright wrong (it's just a restatement of familiarity bias, which generalizes outside music).

[ska]

A fair bit of early music composition is weighted by what is easily sung, more than anything else.