In my opinion, the biggest obstacle for a bill to become a law is the fact that so many different people have to agree on the bill. This makes it pretty tough for any bill to make it into law.
In the process of a bill becoming a law, the members of various committees in both houses have to agree. Then the whole membership of each house has to agree (this is especially difficult in the Senate were on 40% of Senators can stop a bill). If the bills passed by the two houses are different, the conference committee has to agree. Finally, the President has to agree.
At each step of the way, various interest groups will be giving their opinions and they can prevent agreement as well.
So the whole process is a huge obstacle and I will say that the need for so many people to agree is the major reason why passage is difficult.