Yet another way the system works in the interest of the elites is that a high level of conflict in rural areas prevents the periphery from cooperating together against the center. This is hardly unique to Colombia: A common theory of African political dynamics is that the center foments chaos at the periphery in order to "divide and rule." Sudan and Congo are the classic cases.
In most places in the world, one would have thought that either the orangutan would have eventually ripped off the tuxedo and overwhelmed the more functional part of the country, or, that, at some point, the tuxedo would have straitjacketed the orangutan. (The metaphor is strained, but you get the point.) As suggested above, though, disrupting the equilibrium may not be in the interests of those benefitting from the system.
However, the fact that everyone is better off without the ever-present specter of mayhem doesn't guarantee that reason will prevail. I think the real reason for the stability of the Colombian system is that it's largely self-adjusting: The incentives in place are adequate to sustain power-sharing without the need to periodically renegotiate the grand bargain between urban and rural elites.
The answer to why elites on the periphery keep the pot boiling at just the right temperature that denies them dominance over the center is also elusive. The best explanation is that regional elites turn over rapidly, making it difficult to identify collective interest or to act on it when they do.
Rural conflict is further exacerbated by the fact that the ownership of much of the land in Colombia is in dispute, making it difficult to legalize any particular status quo. The rise of the drug cartels since the late 1970s has further complicated conflict resolution, since a lot of illicit drug wealth has gone into acquisition of land (and elite status).