Apparently the letters requesting the illegal aliens to leave the U.S. are not being followed by the recipients.  Worse yet, the follow-up sternly worded letters, which they receive after committing crimes, are also ignored.
The Obama administration is perplexed by the non-compliance…
(Via The Washington Times) Nearly 1 million immigrants are ignoring deportation orders to remain in the U.S. — including more than 170,000 convicted criminals, according to a new report Thursday that suggests the government’s deportation efforts are still falling short.
Only a small fraction of the immigrants are even being detained by Immigration and Customs Enforcement (ICE), meaning most of them remain free on the streets, where they can commit crimes and continue living in the shadows, according to the study by Jessica Vaughan, policy studies director at the Center for Immigration Studies.

“The fact that almost 10 percent of the illegal resident population has already been ordered removed and is still here illustrates just how dysfunctional our immigration enforcement system is. It also should be of great concern that 20 percent of them are conviction criminals, and that most of these are at large in our communities,” Ms. Vaughan said.
She said the 925,193 aliens who were still here despite a deportation order break down into three categories. In some cases their home countries refuse to take them back, and U.S. officials feel constrained by law to release them; other times they are released by sanctuary cities, who help thwart deportations; and still others abscond on their own.
Mexicans account for the most aliens, with nearly 200,000 ignoring deportation orders. About a third of those are convicted criminals, Ms. Vaughan said. El Salvador accounts for more than 150,000 of the aliens, but just 10,000 of them are convicted criminals. (read more)

Oh, ok then… well if only 10,000 of the Salvadorians are convicted criminals…