>>716283823 (OP)A couple of reasons, some of which require some background.
One of the major tenets of the current Way Things Work is that states are the sole proprietors and beneficiaries of violence. Every single time in history a state has failed, it leaves a gap in this monopoly on violence, a gap people like Big Boss can use to challenge the status quo. This extends to the rule of law, which is predicated on the threat of violence (the original bandits and robbers before humans knew what laws were never went away, you see). Failed states, such as contemporary Afghanistan, could not be allowed to exist as areas which have no laws or the ability to enforce them cause problems for all their neighbors.
The time period he became relevant in also plays a part in this. Historically, veterans of wars usually got nothing but a thank you for risking their lives. This led to a lot of social unrest and rebellion, so states like Rome eventually figured out that letting soldiers retire with benefits was a good idea. In the US, Soldiers coming back from World War I, World War II, Korea and Vietnam did not even get a thank you, and had to fight just for what little they did get (check out the Bonus Army for example). Anyone offering to not just reward soldiers for their service but also even put them in charge would be incredibly attractive to the average soldier. Needless to say, the endless warfare this whole system would be predicated on was a raw deal for civilians.
Also the dude had nukes and wasn't shy about his plans to use them. They killed Saddam for less.