except Russian hardware is more reliable ,especially in deserts, there is also something with a price of that weapons and accessibility in black markets in those region where people were using it for years and decades back (usually that is all weapons from army that was taken and those armies had Russian equipement and training). Also, trying to force terrorists to practice on different systems is another problem. Very simple explanation. Similar you can see here:
http://progress-index.com/news/op-ed...hans-1.1522714 Financing is more important then weapons they are using,of course. USA will anyway profit later once new countries are established (training,selling more serious equipement by NATO standards,etc..)