1. CSS1 started modestly by specifying very little over what browsers already did at the time, and from the start locked itself into HTML's inline/block/float/table model.

Back then the old Netscape engine was dominant and it couldn't get even those basics right (http://acid1.acidtests.org), so there was no hope of adoption of a more advanced layout mechanism.

2. CSS had deliberate limitations that did help early implementors: e.g. elements later in the document are not supposed to cause reflow of elements earlier (with tables being "legacy" exception). That's how Opera worked until v6. That what helped all browsers implement progressive rendering. The downside is abysmal vertical centering and limited selectors (flexbox and CSS4 selectors finally drop that limitation).

3. It was hard to get any progress on this for many, many years. First we had Netscape 4, then we had IE6 holding back use of CSS on the web, so specs from that time were mostly untested ivory tower science-fiction.

Things got better since, but of course now we have massive legacy and backward-compatibility constrains... and IE8/9 holding back flexbox.