[5e] Why did they design Legendary creature initiative this way?

Late in 4th edition's design cycle they gave big solo boss monsters multiple initiative counts and had them perform a complete turn each time their initiative came up.

I liked it. It seemed nicely straightforward. I used it a couple of time on homebrew monsters and it didn't cause any problems that I noticed.

Why do you think they went with this much more elaborate "take a limited turn after each player, plus a main turn" paradigm?

[5e] Why did they design Legendary creature initiative this way?