The Obama administration is largely responsible for this. They, along with Durbin, have led a crusade against the for-profit sector with hopes of pushing their "free higher ed" or community college initiatives.
The truth is that U of Phoenix and others met a market need. It is not a first choice for most students, but people who could not attend a traditional university for a variety of reasons (full time work, rural, military) could attend UOPX.
This is a high risk group, but the numbers released on persistence are deceptive. Students with no college experience could try it out for an orientation that lasts a few weeks and others have 4 classes or so to show they can handle it. Many of those drop, and even those that continue have higher risk factors than the traditional student.
Many military students choose it because of the ability to switch between online and ground courses at locations around the world as they are transferred, deployed, etc without losing credits at each move.
The bottom line is that continued attacks on the sector basically take a toll on a free market business that met a need. Those schools may not be your first choice, but for many they are the only reasonable choice.
You'll notice that Durbin, Obama, the DOE and their like never trash community colleges or historically black colleges, many of which have much worse educational persistence than the for-profits.