Loudoun County: 521 square miles
Falls Church City: 2.2 square miles
Fairfax County: 407 square miles
So it isn't land mass.
Median Income 2012
Loudoun County: $118,934
Falls Church City: $121,250
Fairfax County: $106,690
Well that starts to correlate...
http://www.cnsnews.com/news/article/terence-p-jeffrey/census-dc-suburbs-dominate-wealthiest-list-falls-church-va-where-31
Number of Metrorail Stations
Loudoun County: 0 (none)
Fairfax County: 13 (http://www.fairfaxcounty.gov/fcdot/metrorail.htm)
Falls Church City: 0 (none)
There you have it. Higher Access to public transportation and lower median income while still providing a well-thought-of education and access to more jobs (VA/DC/MD) increases rates of ESOL students.
What are they doing? Fewer jobs, less inexpensive public transportation options to those jobs, and higher median income. Though the income thing may just be that you have to be really rich to afford the extra 5% sales tax in Falls Church City, or really rich to afford a car and gas to drive to Fairfax, Arlington, Alexandria or DC or Maryland to get to where the jobs are.
I'd love to see the ESOL students teach other kids to speak other languages. Probably better than the school is teaching the other language now.