Washington's Blog explains why, citing Bill Gross of Pimco, of all people.
If you think about it, the logic is clear.
Austerity measures hurt middle and working class populations, who constitute 80%+ of the population. Higher education costs for college and fewer health and social benefits for populations reduce citizens' consumption power. Consequently, demand for services and products fall.
Austerity doesn't work in most circumstances and 30 years of empirical data drawn from IMF and World Bank imposed austerity in developing nations prove the point.