After the transitional election in 1999, two other general elections have been conducted in 2003 and 2007 respectively. One of the major issues that surrounded those elections was the abusive use of the incumbency factor by those already in power to prevent the alternation of power between the ruling party and the opposition.
IMO, I believe this phenomenon constitutes a serious threat to the process of a democratic consolidation.
Other opinion(s) is/are welcome.

The power of incumbency is a global phenomenom. This includes both credible and non-credible elections. There are several reasons but i'll mention THE MAIN factor i feel is the reason.

Money and Power: Because of the status of the incumbent, he/she has access to money and power/influence. Power/influence is an attraction to lobbyist. In every society the lobbyist are those who control the core mechanisms of the respective area's. For Instance, the Tobbaco company would throw a lot of money to the incumbent to maintain the status quo,Big companies hate drastic changes as it affects their profit margin
.Hence why it is really difficult for a real revolutionary candidate to win an election. Except in a few cases where the incumbent is deeply disliked and the "power that be" cannot influence massive public opinion.