Because too many powers are at play and it's all so messed up, mixed up and tied up that it is impossible to find a solution - because there isn't any.
Take this knot for example, it illustrates only about a tenth of how complicated the situation is fucked up in Lebanon.
Hezbollah: The cancer of Lebanon. Trying to turn Lebanon into an Islamic republic with the Christians' consent - thanks Michel Aoun. This armed militia is stronger than the Lebanese Army and it has created a government within a government in the Chii'a regions. They rule by fear - threatening the country with an invasion every time the government thinks about taking a decision they don't like. They threaten Lebanese peace internally and with Israel - they're the scum of the country. But they're smart.
Michel Aoun & Co.: Loves money and power. Hezbollah gives him money to finance his new TV network (OTV), his advertising campaigns and his party (I guess the 40 million USD he stole from the government before he cowardly went to Paris weren't enough) and power by promising him the presidency (the president of Lebanon has to be Christian Maronite). Michel Aoun & Co. is dragging the country into an abyss because of his folie des grandeurs and his unsatisfied desire to become president of the next Islamic Republic of Lebanon. I wouldn't want to gratify him too much by saying he's the tumor of Lebanon, I'll just say he's a simple but painful wart. Also my dear Michel, if you were Orthodox, Hezbollah wouldn't even care about you.
March 14th: A bunch of useless and ball-less guys decided to get together and name themselves after a date. They put the son of an influent man in power - Saad Hariri - though he is probably the weakest and dumbest politician I've ever seen. Being the son of someone still isn't an acceptable skill in politics - except in the third world. They are trying to make things go forward, but they keep getting interrupted by March 8th - yet another date - which is Hezbollah + Michel Aoun & Co. + some other losers.
Iran: Finances Hezbollah with several billions of dollars a year along with countless weapons and artillery. Iran wants to add Lebanon to its lame-ass Chii'a axis of evil and uses Hezbollah as a serious threat to Israel in case the US try to attack Teheran.
Israel: Could eradicate Hezbollah if it wanted to, but it doesn't really want peace. If it has peaceful relations with Lebanon (Hezbollah), we'd be taking away a very big threat from the Jewish state and they can't justify being in a state of war with half its enemies anymore. And they certainly don't want that because it is this risky situation that attracts all its Jews to the country and "Aid money" (mostly weapon money from Washington). Sure they'll still have problems with the Palestinians - but it's nothing that a giant wall can't solve.
The US: They want to defend Israel no matter what, but they also want to establish a democracy in Lebanon. They're a bit confused as to what to do because everything is so tied up together - well, who wouldn't be. They want to disarm Iran but can't act on it otherwise Hezbollah will attack Israel. They also finance Israel's artillery but help Lebanon with some aid money through USAIDS from the American people (thanks guys!)
Syria: Still can't accept the fact of having lost Lebanon as an occupied territory. They enjoy the sectarian tensions in the country and open their borders to let in all the weapons and money from Iran to Hezbollah allowing an even more unstable situation. This way, Syrians sit back and do nothing while Lebanon is the battleground of all powers. The Syrian-Israeli border hasn't had one clash over the past twenty years - in Lebanon it's a daily thing now. Syria's relations are improving with the US and I think it explains the sudden change of attention from Syria to Hezbollah regarding the Hariri tribunal. The US are being very delicate with Syrians because it's an important geopolitic player in the region.
This is a very superficial analysis, so you can imagine how messed up it is if you had all the details. All this for a piece of land of only 200Km long and 60Km wide with no resources nor political power on the international scene. I don't get it.