The US is generally very reluctant to get involved in the conflict in Syria. There are a number of reasons why some people might want to avoid getting involved. Let us look at some of them.
What happens in Syria does not directly affect the US. Syria cannot directly harm the United States. There is no likelihood that the Assad regime could attack the US. Therefore, the US should not really care about what happens in Syria.
The US cannot really have much influence in Syria. The rebels are not particularly inclined to do what the United States wants. Even if the US gives them money and/or weapons, they still might not turn out to do what the US wants. Therefore, it would be dumb to give them the support.
The US is worried that the weapons that it gives the rebels would end up in the wrong hands. There are definitely some among the rebels who are supporters of Al-Qaeda. The US would not want to give weapons to the rebels and have them end up in the hands of those particular people. It would be hard to prevent this from happening.
These are among the reasons not to get involved.