Larger airliners, like the Airbus's, and 757s, 767s and 747s, have not been brought down because these missiles were not designed to take on aircraft with such large and powerful engines. While these missiles were originally intended for use against jet fighters operating over the battlefield, the reality turned out to be different. The most likely targets encountered were helicopters, or propeller driven transports. These aircraft proved to be just the sort of thing twenty pound missiles with 2-3 pound warheads could destroy. Against jet fighters with powerful engines, the missiles caused some damage to the tailpipe, but usually failed to bring down the jet. This was first noted during the 1973 Arab-Israeli war, where the Egyptians fired hundreds of SA-7s at Israeli A-4 light bombers. Most of the A-4s, with their 11,187 pounds of thrust engines, survived the encounter. Larger jets, like the F-4 and it's 17,000 pound thrust engines, were even more difficult to bring down. Smaller commercial jets, like the 737 or DC-9 (each using two 14,000 pounds of thrust engines) have proved vulnerable. But a 757 has much larger engines with 43,000 pounds of thrust, and the 747 is 63,000. Moreover, the rear end of jet engines are built to take a lot of punishment from all that hot exhaust spewing out. Put a bird into the front of the engine and you can do some real damage. But these missiles home in on heat, and all of that is at the rear end of the engine.
If terrorists target helicopters and smaller turboprop commuter airliners, or business jets, they are likely to take down aircraft better than half the time a missile is used. This takes into account poorly trained missile operators and defective missiles. And a lot of the missile operators will be poorly trained, and, like November, 2002 incident in Mombassa, using missiles built over two decades ago. They won't be using any of the Stingers the U.S. gave out in Afghanistan during the 1980s. The custom battery packs in those missiles gave out in the 1990s. It's a lot easier to get Russian missiles, and fresh batteries for them.
Another option for terrorists is to use anti-tank guided missiles (ATGM) against larger airliners. An ATGM is more accurate and hitting a 747, taking off, in an engine or the main fuel tank between the wings, there is a good chance that the aircraft would crash. But how do you get ATGM? Simple, from one of the Chechen criminal gangs that get all manner of weapons and equipment from light fingered Russian soldiers. The Russians have been cracking down on this illegal arms trade, especially when it comes to surface-to-air missiles. This no doubt was the reason why there was such good cooperation from the Russians in the recent attempt to get a missile into the United States. The Russians knew that if someone was selling SA-18s illegally, the missile would be more likely to be used against Russians than anyone else.
Modern countermeasures for these missiles are mounted on many military aircraft, and some Israeli commercial airliners. These countermeasures consist of a heat detector that is programmed to spot the telltale heat from a launched missile, and then launch a flare that puts out more heat than the engines. But the most modern missiles are programmed to see through this deception. The exact status of the countermeasures versus the most modern missiles in unknown. However, it is known that military aircraft using the countermeasures tend to eject lots of flares, just to be on the safe side. To equip all American commercial aircraft with countermeasures would cost some ten billion dollars (higher ticket prices), plus hundreds of millions a year to maintain them (and more flight delays because it's another piece of equipment that could fail selftest before takeoff.) Another problem is that these countermeasures would have more "false positives" than actual use, meaning there would be problems with flares coming down in inhabited areas, or dry forests and starting fires.
There are several hundred thousand portable surface-to-air missiles out there, but most of them are older models like the SA-7. Many of these are defective from old age, or rough treatment. This is known because of the consistently poor performance of these older missiles.
Also keep in mind that, with all those missiles out there, and so many terrorists (not just Islamic ones) eager to use them against civilians, very few are used. Many of the terrorists know that most of those SA-7s are crap, and that explains why they are looking for the harder to get modern missiles.
An Indian arms dealer was caught trying to illegally import a Russian SA-18 Igla shoulder fired anti-aircraft missile to an FBI agent posing as an Islamic terrorist. Terrorists trying to take down airliners with portable missiles has been a threat for a long time. Actually, over the last thirty years, it's been a reality. Some 29 commercial aircraft have been shot down by such missiles. However, the downed aircraft have been small, and most of these tragedies have taken place in Africa. The wars in Africa are the worst on the planet, so violent that most journalists avoid them. For three decades, this has kept the use of portable missiles against civilian aircraft off the front page.