I always thought the SOC eventually did accept the last five books into their canon. Maybe one of our Syriac Orthodox brothers could comment on this, as well as whether they are read liturgically.
As to why there are differences in canons, I think the development of Biblical canons was less "organized" than most people think. The councils that decreed the canon of 27 books were not even ecumenical in nature, but rather local. The acceptance of that canon just sort of happened over time. The Armenians, for example, for a long time had Third Corinthians, and didn't have Revelation until about one thousand years ago. People who lived on the edge of, or outside of, the empire, would have taken longer to accept all 27. Communication took a lot longer, and of course you didn't have printing presses. Things just happened more slowly back then.