Because already popular posts tend to get more popular and that encourages more engagement, and for the advertisers engagement (likes/comments) are far more important than accuracy or completeness.
I can see why the default would be an algorithmic timeline, but for the small subset of people that want the chronological, just give it to them.
Whatever happened to the idea of delighting your users?
FB has always been ignoring users (kinda like Firefox!).
See the original "wall" and the system before "timeline".