score:5
The main thing that the Central American Republic going for it was its fear of Mexico. Otherwise, the countries had rather little in common.
The fear of Mexico diminished in the late 1830s, after Santa Anna was defeated by the Texans in the (1836) War of Independence. It might not be an accident that the Central American Republic started to dissolve around 1840. Ditto for the fact that the first countries to leave, Costa Rica and Nicaragua were the furthest away from Mexico, as well as being the most prosperous. (Panama was then part of Colombia, not part of the Central American Federation.)
Upvote:2
The Federal Republic of Central America was not a stable country because the area it contained was very difficult to rule in a unified way. This was for several reasons:
Under these conditions it was difficult to govern with consensus. Even recently there have been many insurgency groups in these countries and little civil wars going on. It is hard enough just keeping Guatemala in one piece, much less the whole region.