Singer wanted to reform moral code so that people would be more giving and less selfish. If everyone acted according to his principles the world would be a better place (there would be less suffering). In a utopian society his principles would thrive, but we live in a world where people benefit from exploitation of others. If everyone was willing to sacrifice at the level Singer expects then there would be an abundance of people prepared to sacrifice to others in need. If this became reality then some people would be incentivized to act lazily and expect others to provide for them.
This scenario is very similar to communism. It sounds great that everyone should share resources equally but in reality it does not work. People only act when their backs are up-against the wall. If the government/society provides too big of a safety net then people will have no incentive to provide for themselves by working hard.
In America, our current moral code works because it creates competition. Individuals have a right to their property and are only required to give away a small part of it through taxation. Because of this, people are incentivized to accumulate as much wealth as possible. This encourages competition in the economy. Competition is important because people are striving to innovate their products and invent things that are valued by society.
So, if we abide by Singer's principles then innovation and invention would not be incentivized in our society. It is moral to help others in need, but forcing people to help others would create an economic standstill that would lead to more problems than it solves.