History of privAtization
Legislation first allowed private space companies to exist in 2004. Since then, they have risen in popularity for things like resupply missions and asteroid mining.
Their popularity rose when Congress slashed NASA's budget by about half. While NASA still wanted to explore, they simply didn't have enough money to go anywhere.
Several private companies have been created to fill the void created by Nasa's absence. These companies usually assist NASA; for example, they have resupplied the ISS since NASA was forced to cancel their doing it.
Private space companies tend to be cheaper than their government counterparts. For example, NASA's resupply missions had an annual cost of 4 billion dollars, the new missions cost $50 million per launch. This allows NASA to spend more time and money on other projects; like the development of long-term space travel.
Although space privatization is good news for government companies, it isn't for the private companies themselves. Many companies have a serious problem where they promise a lot and then deliver little. Also, these companies have to deal with things such as profitability and contract security. NASA doesn't have to worry about these things because they are funded directly by the US government. Companies also may not have the funds or experience, or even interest to explore space. However, this is not the same in countries like Russia and China, whose governments more heavily fund space exploration than the US.